![Go Programming
35
Line 1 - Type of variable a = int
Line 2 - Type of variable b = int32
Line 3 - Type of variable c= float32
value of a is 4
*ptr is 4.
OperatorsPrecedenceinGo
Operator precedence determines the grouping of terms in an expression. This
affects how an expression is evaluated. Certain operators have higher precedence
than others; for example, the multiplication operator has higher precedence than
the addition operator.
For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has
higher precedence than +, so it first gets multiplied with 3*2 and then adds into
7.
Here, operators with the highest precedence appear at the top of the table, and
those with the lowest appear at the bottom. Within an expression, higher
precedence operators will be evaluated first.
Category Operator Associativity
Postfix () [] -> . ++ - - Left to right
Unary + - ! ~ ++ - - (type)* & sizeof Right to left
Multiplicative * / % Left to right
Additive + - Left to right
Shift << >> Left to right
Relational < <= > >= Left to right
Equality == != Left to right
Bitwise AND & Left to right
Bitwise XOR ^ Left to right
Bitwise OR | Left to right
Logical AND && Left to right](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-42-320.jpg)
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Syntax
The syntax of for loop in Go programming language is:
for [condition | ( init; condition; increment ) | Range]
{
statement(s);
}
The flow of control in a for loop is as follows:
If a condition is available, then for loop executes as long as condition is
true.
If a for clause that is ( init; condition; increment ) is present, then:
o The init step is executed first, and only once. This step allows you to
declare and initialize any loop control variables. You are not required to
put a statement here, as long as a semicolon appears.
o Next, the condition is evaluated. If it is true, the body of the loop is
executed. If it is false, the body of the loop does not execute and the
flow of control jumps to the next statement just after the for loop.
o After the body of the for loop executes, the flow of control jumps back
up to the increment statement. This statement allows you to update
any loop control variables. This statement can be left blank, as long as
a semicolon appears after the condition.
o The condition is now evaluated again. If it is true, the loop executes and
the process repeats itself (body of loop, then increment step, and then
again the condition). After the condition becomes false, the for loop
terminates.
If range is available, then the for loop executes for each item in the range.](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-60-320.jpg)
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Flow Diagram
Example
package main
import "fmt"
func main() {
var b int = 15
var a int
numbers := [6]int{1, 2, 3, 5}](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-61-320.jpg)
![Go Programming
56
value of a: 8
value of a: 9
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of x = 1 at 0
value of x = 2 at 1
value of x = 3 at 2
value of x = 5 at 3
value of x = 0 at 4
value of x = 0 at 5
NestedforLoops
Go programming language allows to use one loop inside another loop. The
following section shows a few examples to illustrate the concept:
Syntax
The syntax for a nested for loop statement in Go is as follows:
for [condition | ( init; condition; increment ) | Range]
{
for [condition | ( init; condition; increment ) | Range]
{
statement(s);
}
statement(s);
}](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-63-320.jpg)
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66
A function is a group of statements that together perform a task. Every Go
program has at least one function, which is main(). You can divide your code into
separate functions. How you divide your code among different functions is up to
you, but logically, the division should be such that each function performs a
specific task.
A function declaration tells the compiler about a function name, return type, and
parameters. A function definition provides the actual body of the function.
The Go standard library provides numerous built-in functions that your program
can call. For example, the function len() takes arguments of various types and
returns the length of the type. If a string is passed to it, the function returns the
length of the string in bytes. If an array is passed to it, the function returns the
length of the array.
Functions are also known as method, sub-routine, or procedure.
DefiningaFunction
The general form of a function definition in Go programming language is as
follows:
func function_name( [parameter list] ) [return_types]
{
body of the function
}
A function definition in Go programming language consists of a function header and
a function body. Here are all the parts of a function:
Func: It starts the declaration of a function.
Function Name: It is the actual name of the function. The function name
and the parameter list together constitute the function signature.
Parameters: A parameter is like a placeholder. When a function is invoked,
you pass a value to the parameter. This value is referred to as actual
parameter or argument. The parameter list refers to the type, order, and
number of the parameters of a function. Parameters are optional; that is, a
function may contain no parameters.
Return Type: A function may return a list of values. The return_types is
the list of data types of the values the function returns. Some functions
perform the desired operations without returning a value. In this case, the
return_type is the not required.
11.FUNCTIONS](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-73-320.jpg)
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fmt.Println(nextNumber())
fmt.Println(nextNumber())
/* create a new sequence and see the result, i is 0 again*/
nextNumber1 := getSequence()
fmt.Println(nextNumber1())
fmt.Println(nextNumber1())
}
When the above code is compiled and executed, it produces the following result:
1
2
3
1
2
Method
Go programming language supports special types of functions called methods. In
method declaration syntax, a "receiver" is present to represent the container of
the function. This receiver can be used to call a function using "." operator. For
example:
Syntax
func (variable_name variable_data_type) function_name() [return_type]{
/* function body*/
}
package main
import (
"fmt"
"math"
)
/* define a circle */
type Circle strut {](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-82-320.jpg)
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81
Strings, which are widely used in Go programming, are a read-only slice of bytes.
In the Go programming language, strings are slices. The Go platform provides
various libraries to manipulate strings:
unicode
regexp
strings
CreatingStrings
The most direct way to create a string is to write:
var greeting = "Hello world!"
Whenever it encounters a string literal in your code, the compiler creates a string
object with its value. In this case, it is "Hello world!'.
A string literal holds valid UTF-8 sequences called runes. A string holds arbitrary
bytes.
package main
import "fmt"
func main() {
var greeting = "Hello world!"
fmt.Printf("normal string: ")
fmt.Printf("%s", greeting)
fmt.Printf("n")
fmt.Printf("hex bytes: ")
for i := 0; i < len(greeting); i++ {
fmt.Printf("%x ", greeting[i])
}
fmt.Printf("n")
const sampleText = "xbdxb2x3dxbcx20xe2x8cx98"
13.STRINGS](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-88-320.jpg)
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Let us look at the following example:
package main
import (
"fmt"
"strings"
)
func main() {
greetings := []string{"Hello","world!"}
fmt.Println(strings.Join(greetings, " "))
}
It would produce the following result:
Hello world!](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-90-320.jpg)
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84
Go programming language provides a data structure called the array, which can
store a fixed-size sequential collection of elements of the same data type. An array
is used to store a collection of data, but it is often more useful to think of an array
as a collection of variables of the same type.
Instead of declaring individual variables, such as number0, number1, ...,
number99, you declare one array variable such as numbers and use numbers[0],
numbers[1], and ..., numbers[99] to represent individual variables. A specific
element in an array is accessed by an index.
All arrays consist of contiguous memory locations. The lowest address corresponds
to the first element and the highest address to the last element.
DeclaringArrays
To declare an array in Go, a programmer specifies the type of the elements and
the number of elements required by an array as follows:
var variable_name [SIZE] variable_type
This is called a single-dimensional array. The arraySize must be an integer
constant greater than zero and type can be any valid Go data type. For example,
to declare a 10-element array called balance of type float32, use this statement:
var balance [10] float32
Here, balance is a variable array that can hold up to 10 float numbers.
InitializingArrays
You can initialize an array in Go, either one by one or using a single statement as
follows:
var balance = [5]float32{1000.0, 2.0, 3.4, 7.0, 50.0}
14.ARRAYS](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-91-320.jpg)
![Go Programming
85
The number of values between the braces { } cannot be larger than the number
of elements that we declare for the array between the square brackets [ ].
If you omit the size of the array, an array just big enough to hold the initialization
is created. Therefore, if you write:
var balance = []float32{1000.0, 2.0, 3.4, 7.0, 50.0}
You will create exactly the same array as you did in the previous example.
Following is an example to assign a single element of the array:
balance[4] = 50.0
The above statement assigns element number 5th in the array with a value of
50.0. All arrays have 0 as the index of their first element which is also called base
index and last index of an array will be total size of the array minus 1. Following
is the pictorial representation of the same array we discussed above:
AccessingArrayElements
An element is accessed by indexing the array name. This is done by placing the
index of the element within square brackets after the name of the array. For
example:
float32 salary = balance[9]
The above statement will take the 10th element from the array and assign the
value to salary variable. Following is an example which will use all the above-
mentioned three concepts, viz. declaration, assignment, and accessing arrays:
package main
import "fmt"
func main() {
var n [10]int /* n is an array of 10 integers */
var i,j int
/* initialize elements of array n to 0 */
for i = 0; i < 10; i++ {](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-92-320.jpg)
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86
n[i] = i + 100 /* set element at location i to i + 100 */
}
/* output each array element's value */
for j = 0; j < 10; j++ {
fmt.Printf("Element[%d] = %dn", j, n[j] )
}
}
When the above code is compiled and executed, it produces the following result:
Element[0] = 100
Element[1] = 101
Element[2] = 102
Element[3] = 103
Element[4] = 104
Element[5] = 105
Element[6] = 106
Element[7] = 107
Element[8] = 108
Element[9] = 109
GoArraysinDetail
There are important concepts related to array which should be clear to a Go
programmer:
Concept Description
Multi-dimensional arrays
Go supports multidimensional arrays. The
simplest form of a multidimensional array is the
two-dimensional array.
Passing arrays to functions
You can pass to the function a pointer to an array
by specifying the array's name without an index.](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-93-320.jpg)
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MultidimensionalArraysinGo
Go programming language allows multidimensional arrays. Here is the general
form of a multidimensional array declaration:
var variable_name [SIZE1][SIZE2]...[SIZEN] variable_type
For example, the following declaration creates a three-dimensional [5][10][4]
integer array:
var threedim [5][10][4]int
Two-DimensionalArrays
A two-dimensional array is the simplest form of a multidimensional array. A two-
dimensional array is, in essence, a list of one-dimensional arrays. To declare a
two-dimensional integer array of size [x, y], you would write something as follows:
var arrayName [ x ][ y ] variable_type
Where variable_type can be any valid Go data type and arrayName can be a
valid Go identifier. A two-dimensional array can be taken as a table having x
number of rows and y number of columns. A 2-dimensional array a, which contains
three rows and four columns can be shown as below:
Thus, every element in the array a is identified by an element name as a[i][j],
where a is the name of the array, and i and j are the subscripts that uniquely
identify each element in a.
InitializingTwo-DimensionalArrays
Multidimensional arrays may be initialized by specifying bracketed values for each
row. The following array is with 3 rows and each row has 4 columns.
a = [3][4]int{
{0, 1, 2, 3} , /* initializers for row indexed by 0 */
{4, 5, 6, 7} , /* initializers for row indexed by 1 */](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-94-320.jpg)
![Go Programming
88
{8, 9, 10, 11} /* initializers for row indexed by 2 */
}
AccessingTwo-DimensionalArrayElements
An element in a two-dimensional array is accessed by using the subscripts, i.e.,
row index and column index of the array. For example:
int val = a[2][3]
The above statement takes the 4th element from the 3rd row of the array. You
can verify it in the above diagram. Let us check the following program where we
have used nested loop to handle a two-dimensional array:
package main
import "fmt"
func main() {
/* an array with 5 rows and 2 columns*/
var a = [5][2]int{ {0,0}, {1,2}, {2,4}, {3,6},{4,8}}
var i, j int
/* output each array element's value */
for i = 0; i < 5; i++ {
for j = 0; j < 2; j++ {
fmt.Printf("a[%d][%d] = %dn", i,j, a[i][j] )
}
}
}
When the above code is compiled and executed, it produces the following result:
a[0][0]: 0
a[0][1]: 0
a[1][0]: 1
a[1][1]: 2
a[2][0]: 2
a[2][1]: 4](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-95-320.jpg)
![Go Programming
89
a[3][0]: 3
a[3][1]: 6
a[4][0]: 4
a[4][1]: 8
As explained above, you can have arrays with any number of dimensions, although
it is likely that most of the arrays you create will be of one or two dimensions.
PassingArraystoFunctions
If you want to pass a one-dimensional array as an argument in a function, you
would have to declare a function formal parameter in one of following two ways.
Both the declaration methods produce similar results because each tells the
compiler that an integer array is going to be received. Similarly, you can pass a
multidimensional array as a formal parameter.
Way-1
Formal parameters as a sized array as follows:
void myFunction(param [10]int)
{
.
.
.
}
Way-2
Formal parameters as an unsized array as follows:
void myFunction(param []int)
{
.
.
.
}](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-96-320.jpg)
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90
Example
Now, consider the following function, which will take an array as an argument
along with another argument and based on the passed arguments, it will return
the average of the numbers passed through the array as follows:
func getAverage(arr []int, int size) float32
{
var i int
var avg, sum float32
for i = 0; i < size; ++i {
sum += arr[i]
}
avg = sum / size
return avg;
}
Now, let us call the above function as follows:
package main
import "fmt"
func main() {
/* an int array with 5 elements */
var balance = []int {1000, 2, 3, 17, 50}
var avg float32
/* pass array as an argument */
avg = getAverage( balance, 5 ) ;
/* output the returned value */
fmt.Printf( "Average value is: %f ", avg );
}
func getAverage(arr []int, size int) float32 {](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-97-320.jpg)
![Go Programming
91
var i,sum int
var avg float32
for i = 0; i < size;i++ {
sum += arr[i]
}
avg = float32(sum / size)
return avg;
}
When the above code is compiled together and executed, it produces the following
result:
Average value is: 214.400000
As you can see, the length of the array doesn't matter as far as the function is
concerned because Go performs no bounds checking for formal parameters.](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-98-320.jpg)
![Go Programming
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Concept Description
Go – Array of pointers
You can define arrays to hold a number
of pointers.
Go – Pointer to pointer
Go allows you to have pointer on a
pointer and so on.
Passing pointers to functions in Go
Passing an argument by reference or by
address both enable the passed
argument to be changed in the calling
function by the called function.
Go–ArrayofPointers
Before we understand the concept of arrays of pointers, let us consider the
following example, which makes use of an array of 3 integers:
package main
import "fmt"
const MAX int = 3
func main() {
a := []int{10,100,200}
var i int
for i = 0; i < MAX; i++ {
fmt.Printf("Value of a[%d] = %dn", i, a[i] )
}
}
When the above code is compiled and executed, it produces the following result:
Value of a[0] = 10
Value of a[1] = 100
Value of a2] = 200](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-102-320.jpg)
![Go Programming
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There may be a situation when we want to maintain an array, which can store
pointers to an int or string or any other data type available. The following
statement declares an array of pointers to an integer:
var ptr [MAX]*int;
This declares ptr as an array of MAX integer pointers. Thus, each element in ptr,
now holds a pointer to an int value. The following example makes use of three
integers, which will be stored in an array of pointers as follows:
package main
import "fmt"
const MAX int = 3
func main() {
a := []int{10,100,200}
var i int
var ptr [MAX]*int;
for i = 0; i < MAX; i++ {
ptr[i] = &a[i] /* assign the address of integer. */
}
for i = 0; i < MAX; i++ {
fmt.Printf("Value of a[%d] = %dn", i,*ptr[i] )
}
}
When the above code is compiled and executed, it produces the following result:
Value of a[0] = 10
Value of a[1] = 100
Value of a[2] = 200
Go–PointertoPointer
A pointer to a pointer is a form of chain of pointers. Normally, a pointer contains
the address of a variable. When we define a pointer to a pointer, the first pointer
contains the address of the second pointer, which points to the location that
contains the actual value as shown below.](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-103-320.jpg)
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106
Go Slice is an abstraction over Go Array. Go Array allows you to define variables
that can hold several data items of the same kind but it does not provide any
inbuilt method to increase its size dynamically or get a sub-array of its own. Slices
overcome this limitation. It provides many utility functions required on Array and
is widely used in Go programming.
Definingaslice
To define a slice, you can declare it as an array without specifying its size.
Alternatively, you can use make function to create a slice.
var numbers []int /* a slice of unspecified size */
/* numbers == []int{0,0,0,0,0}*/
numbers = make([]int,5,5) /* a slice of length 5 and capacity 5*/
len()andcap()functions
A slice is an abstraction over array. It actually uses arrays as an underlying
structure. The len() function returns the elements presents in the slice
where cap() function returns the capacity of the slice (i.e., how many elements it
can be accommodate). The following example explains the usage of slice:
package main
import "fmt"
func main {
var numbers = make([]int,3,5)
printSlice(numbers)
}
func printSlice(x []int){
fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x)
}
When the above code is compiled and executed, it produces the following result:
17.SLICES](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-113-320.jpg)
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len=3 cap=5 slice=[0 0 0]
Nilslice
If a slice is declared with no inputs, then by default, it is initialized as nil. Its length
and capacity are zero. For example:
package main
import "fmt"
func main {
var numbers []int
printSlice(numbers)
if(numbers == nil){
fmt.printf("slice is nil")
}
}
func printSlice(x []int){
fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x)
}
When the above code is compiled and executed, it produces the following result:
len=0 cap=0 slice=[]
slice is nil
Subslicing
Slice allows lower-bound and upper bound to be specified to get its subslice using
[lower-bound:upper-bound]. For example:](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-114-320.jpg)
![Go Programming
108
package main
import "fmt"
func main {
/* create a slice */
numbers := []int{0,1,2,3,4,5,6,7,8}
printSlice(numbers)
/* print the original slice */
fmt.Println("numbers ==", numbers)
/* print the sub slice starting from index 1(included) to index
4(excluded)*/
fmt.Println("numbers[1:4] ==", numbers[1:4])
/* missing lower bound implies 0*/
fmt.Println("numbers[:3] ==", numbers[:3])
/* missing upper bound implies len(s)*/
fmt.Println("numbers[4:] ==", numbers[4:])
numbers1 := make([]int,0,5)
printSlice(numbers1)
/* print the sub slice starting from index 0(included) to index
2(excluded) */
number2 := numbers[:2]
printSlice(number2)
/* print the sub slice starting from index 2(included) to index
5(excluded) */
number3 := numbers[2:5]
printSlice(number3)
}](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-115-320.jpg)
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func printSlice(x []int){
fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x)
}
When the above code is compiled and executed, it produces the following result:
len=9 cap=9 slice=[0 1 2 3 4 5 6 7 8]
numbers == [0 1 2 3 4 5 6 7 8]
numbers[1:4] == [1 2 3]
numbers[:3] == [0 1 2]
numbers[4:] == [4 5 6 7 8]
len=0 cap=5 slice=[]
len=2 cap=9 slice=[0 1]
len=3 cap=7 slice=[2 3 4]
append()andcopy()Functions
One can increase the capacity of a slice using the append() function.
Using copy() function, the contents of a source slice are copied to a destination
slice. For example:
package main
import "fmt"
func main {
var numbers []int
printSlice(numbers)
/* append allows nil slice */
numbers = append(numbers, 0)
printSlice(numbers)
/* add one element to slice*/
numbers = append(numbers, 1)
printSlice(numbers)](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-116-320.jpg)
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110
/* add more than one element at a time*/
numbers = append(numbers, 2,3,4)
printSlice(numbers)
/* create a slice numbers1 with double the capacity of earlier slice*/
numbers1 := make([]int, len(numbers), (cap(numbers))*2)
/* copy content of numbers to numbers1 */
copy(numbers1,numbers)
printSlice(numbers1)
}
func printSlice(x []int){
fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x)
}
When the above code is compiled and executed, it produces the following result:
len=0 cap=0 slice=[]
len=1 cap=2 slice=[0]
len=2 cap=2 slice=[0 1]
len=5 cap=8 slice=[0 1 2 3 4]
len=5 cap=16 slice=[0 1 2 3 4]](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-117-320.jpg)
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The range keyword is used in for loop to iterate over items of an array, slice,
channel or map. With arrays and slices, it returns the index of the item as integer.
With maps, it returns the key of the next key-value pair. Range either returns one
value or two. If only one value is used on the left of a range expression, it is the
1st value in the following table.
Range expression 1st Value 2nd Value(Optional)
Array or slice a [n]E index i int a[i] E
String s string type index i int rune int
map m map[K]V key k K value m[k] V
channel c chan E element e E none
Example
The following paragraph shows how to use range:
package main
import "fmt"
func main {
/* create a slice */
numbers := []int{0,1,2,3,4,5,6,7,8}
/* print the numbers */
for i:= range numbers {
fmt.Println("Slice item",i,"is",numbers[i])
}
/* create a map*/
coutryCapitalMap := map[string] string
{"France":"Paris","Italy":"Rome","Japan":"Tokyo"}
18.RANGE](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-118-320.jpg)
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/* print map using keys*/
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}
/* print map using key-value*/
for country,capital := range countryCapitalMap {
fmt.Println("Capital of",country,"is",capital)
}
}
When the above code is compiled and executed, it produces the following result:
Slice item 0 is 0
Slice item 1 is 1
Slice item 2 is 2
Slice item 3 is 3
Slice item 4 is 4
Slice item 5 is 5
Slice item 6 is 6
Slice item 7 is 7
Slice item 8 is 8
Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-119-320.jpg)
![Go Programming
113
Go provides another important data type named map which maps unique keys to
values. A key is an object that you use to retrieve a value at a later date. Given a
key and a value, you can store the value in a Map object. After the value is stored,
you can retrieve it by using its key.
DefiningaMap
You must use make function to create a map.
/* declare a variable, by default map will be nil*/
var map_variable map[key_data_type]value_data_type
/* define the map as nil map can not be assigned any value*/
map_variable = make(map[key_data_type]value_data_type)
Example
The following example illustrates how to create and use a map:
package main
import "fmt"
func main {
var coutryCapitalMap map[string]string
/* create a map*/
coutryCapitalMap = make(map[string]string)
/* insert key-value pairs in the map*/
countryCapitalMap["France"] = "Paris"
countryCapitalMap["Italy"] = "Rome"
countryCapitalMap["Japan"] = "Tokyo"
countryCapitalMap["India"] = "New Delhi"
/* print map using keys*/
19.MAPS](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-120-320.jpg)
![Go Programming
114
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}
/* test if entry is present in the map or not*/
captial, ok := countryCapitalMap["United States"]
/* if ok is true, entry is present otherwise entry is absent*/
if(ok){
fmt.Println("Capital of United States is", capital)
}else {
fmt.Println("Capital of United States is not present")
}
}
When the above code is compiled and executed, it produces the following result:
Capital of India is New Delhi
Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of United States is not present
delete()Function
delete() function is used to delete an entry from a map. It requires the map and
the corresponding key which is to be deleted. For example:
package main
import "fmt"
func main {
/* create a map*/
coutryCapitalMap := map[string] string
{"France":"Paris","Italy":"Rome","Japan":"Tokyo","India":"New Delhi"}
fmt.Println("Original map")](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-121-320.jpg)
![Go Programming
115
/* print map */
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}
/* delete an entry */
delete(countryCapitalMap,"France");
fmt.Println("Entry for France is deleted")
fmt.Println("Updated map")
/* print map */
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}
}
When the above code is compiled and executed, it produces the following result:
Original Map
Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of India is New Delhi
Entry for France is deleted
Updated Map
Capital of India is New Delhi
Capital of Italy is Rome
Capital of Japan is Tokyo](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-122-320.jpg)
![Go Programming
120
Go programming provides another data type called interfaces which represents
a set of method signatures. The struct data type implements these interfaces to
have method definitions for the method signature of the interfaces.
Syntax
/* define an interface */
type interface_name interface {
method_name1 [return_type]
method_name2 [return_type]
method_name3 [return_type]
...
method_namen [return_type]
}
/* define a struct */
type struct_name struct {
/* variables */
}
/* implement interface methods*/
func (struct_name_variable struct_name) method_name1() [return_type] {
/* method implementation */
}
...
func (struct_name_variable struct_name) method_namen() [return_type] {
/* method implementation */
}
22.INTERFACES](https://image.slidesharecdn.com/gotutorial-180215075816/85/Go-tutorial-127-320.jpg)
Go tutorial
- 2. Go Programming i AbouttheTutorial Go language is a programming language initially developed at Google in the year 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. It is a statically-typed language having syntax similar to that of C. It provides garbage collection, type safety, dynamic-typing capability, many advanced built-in types such as variable length arrays and key-value maps. It also provides a rich standard library. The Go programming language was launched in November 2009 and is used in some of the Google's production systems. Audience This tutorial is designed for software programmers with a need to understand the Go programming language from scratch. This tutorial will give you enough understanding on Go programming language from where you can take yourself to higher levels of expertise. Prerequisites Before proceeding with this tutorial, you should have a basic understanding of computer programming terminologies. If you have a good command over C, then it would be quite easy for you to understand the concepts of Go programming and move fast on the learning track. Disclaimer&Copyright Copyright 2015 by Tutorials Point (I) Pvt. Ltd. All the content and graphics published in this e-book are the property of Tutorials Point (I) Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish any contents or a part of contents of this e-book in any manner without written consent of the publisher. We strive to update the contents of our website and tutorials as timely and as precisely as possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt. Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our website or its contents including this tutorial. If you discover any errors on our website or in this tutorial, please notify us at [email protected].
- 3. Go Programming ii TableofContents About the Tutorial .....................................................................................................................................i Audience....................................................................................................................................................i Prerequisites..............................................................................................................................................i Table of Contents......................................................................................................................................ii 1. OVERVIEW ............................................................................................................................1 Features of Go Programming ....................................................................................................................1 Features Excluded Intentionally................................................................................................................1 Go Programs .............................................................................................................................................2 Compiling and Executing Go Programs......................................................................................................2 2. ENVIRONMENT SETUP ..........................................................................................................3 Try it Option Online ..................................................................................................................................3 Local Environment Setup ..........................................................................................................................3 Text Editor ................................................................................................................................................3 The Go Compiler .......................................................................................................................................4 Download Go Archive ...............................................................................................................................4 Installation on UNIX/Linux/Mac OS X, and FreeBSD .................................................................................4 Installation on Windows ...........................................................................................................................5 Verifying the Installation...........................................................................................................................5 3. PROGRAM STRUCTURE.........................................................................................................6 Hello World Example ................................................................................................................................6 Executing a Go Program............................................................................................................................7 4. BASIC SYNTAX .......................................................................................................................8 Tokens in Go .............................................................................................................................................8 Line Separator...........................................................................................................................................8 Comments ................................................................................................................................................8
- 4. Go Programming iii Identifiers .................................................................................................................................................9 Keywords..................................................................................................................................................9 Whitespace in Go......................................................................................................................................9 5. DATA TYPES ........................................................................................................................11 Integer Types ..........................................................................................................................................12 Floating Types.........................................................................................................................................12 Other Numeric Types ..............................................................................................................................13 6. VARIABLES ..........................................................................................................................14 Variable Definition in Go.........................................................................................................................14 Static Type Declaration in Go..................................................................................................................15 Dynamic Type Declaration / Type Inference in Go ..................................................................................16 Mixed Variable Declaration in Go ...........................................................................................................16 The lvalues and the rvalues in Go............................................................................................................17 7. CONSTANTS ........................................................................................................................19 Integer Literals........................................................................................................................................19 Floating-point Literals .............................................................................................................................20 Escape Sequence.....................................................................................................................................20 String Literals in Go.................................................................................................................................21 The const Keyword..................................................................................................................................22 8. OPERATORS ........................................................................................................................23 Arithmetic Operators..............................................................................................................................23 Relational Operators...............................................................................................................................25 Logical Operators....................................................................................................................................27 Bitwise Operators ...................................................................................................................................29 Assignment Operators ............................................................................................................................31 Miscellaneous Operators ........................................................................................................................34
- 5. Go Programming iv Operators Precedence in Go ...................................................................................................................35 9. DECISION MAKING..............................................................................................................38 The if Statement .....................................................................................................................................39 The if…else Statement ............................................................................................................................40 Nested if Statement................................................................................................................................42 The Switch Statement.............................................................................................................................43 The Select Statement ..............................................................................................................................48 The if...else if...else Statement................................................................................................................49 10. LOOPS.................................................................................................................................52 for Loop ..................................................................................................................................................52 Nested for Loops.....................................................................................................................................56 Loop Control Statements ........................................................................................................................58 The continue Statement..........................................................................................................................60 The goto Statement................................................................................................................................62 The Infinite Loop.....................................................................................................................................64 11. FUNCTIONS.........................................................................................................................66 Defining a Function.................................................................................................................................66 Calling a Function....................................................................................................................................67 Returning Multiple Values from Function ...............................................................................................68 Function Arguments................................................................................................................................69 Call by Value ...........................................................................................................................................70 Call by Reference ....................................................................................................................................71 Function Usage .......................................................................................................................................73 Function Closures....................................................................................................................................74 Method...................................................................................................................................................75
- 6. Go Programming v 12. SCOPE RULES ......................................................................................................................77 Local Variables........................................................................................................................................77 Global Variables......................................................................................................................................78 Formal Parameters .................................................................................................................................79 Initializing Local and Global Variables.....................................................................................................80 13. STRINGS..............................................................................................................................81 Creating Strings.......................................................................................................................................81 String Length...........................................................................................................................................82 Concatenating Strings .............................................................................................................................82 14. ARRAYS ...............................................................................................................................84 Declaring Arrays......................................................................................................................................84 Initializing Arrays ....................................................................................................................................84 Accessing Array Elements .......................................................................................................................85 Go Arrays in Detail..................................................................................................................................86 Multidimensional Arrays in Go ...............................................................................................................87 Two-Dimensional Arrays.........................................................................................................................87 Initializing Two-Dimensional Arrays........................................................................................................87 Accessing Two-Dimensional Array Elements ...........................................................................................88 Passing Arrays to Functions ....................................................................................................................89 15. POINTERS............................................................................................................................92 What Are Pointers?.................................................................................................................................92 How to Use Pointers?..............................................................................................................................93 Nil Pointers in Go....................................................................................................................................94 Go Pointers in Detail...............................................................................................................................94 Go – Array of Pointers.............................................................................................................................95 Go – Pointer to Pointer ...........................................................................................................................96 Go – Passing Pointers to Functions .........................................................................................................98
- 7. Go Programming vi 16. STRUCTURES.....................................................................................................................100 Defining a Structure..............................................................................................................................100 Accessing Structure Members...............................................................................................................100 Structures as Function Arguments ........................................................................................................102 Pointers to Structures ...........................................................................................................................104 17. SLICES ...............................................................................................................................106 Defining a slice......................................................................................................................................106 len() and cap() functions .......................................................................................................................106 Nil slice .................................................................................................................................................107 Subslicing..............................................................................................................................................107 append() and copy() Functions..............................................................................................................109 18. RANGE ..............................................................................................................................111 19. MAPS ................................................................................................................................113 Defining a Map .....................................................................................................................................113 delete() Function...................................................................................................................................114 20. RECURSION.......................................................................................................................116 Examples of Recursion in Go.................................................................................................................116 21. TYPE CASTING...................................................................................................................119 22. INTERFACES ......................................................................................................................120 23. ERROR HANDLING.............................................................................................................123
- 8. Go Programming 1 Go is a general-purpose language designed with systems programming in mind. It was initially developed at Google in the year 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. It is strongly and statically typed, provides inbuilt support for garbage collection, and supports concurrent programming. Programs are constructed using packages, for efficient management of dependencies. Go programming implementations use a traditional compile and link model to generate executable binaries. The Go programming language was announced in November 2009 and is used in some of the Google's production systems. FeaturesofGoProgramming The most important features of Go programming are listed below: Support for environment adopting patterns similar to dynamic languages. For example, type inference (x := 0 is valid declaration of a variable x of type int) Compilation time is fast. Inbuilt concurrency support: lightweight processes (via go routines), channels, select statement. Go programs are simple, concise, and safe. Support for Interfaces and Type embedding. Production of statically linked native binaries without external dependencies. FeaturesExcludedIntentionally To keep the language simple and concise, the following features commonly available in other similar languages are omitted in Go: Support for type inheritance Support for method or operator overloading Support for circular dependencies among packages Support for pointer arithmetic Support for assertions Support for generic programming 1. OVERVIEW
- 9. Go Programming 2 GoPrograms A Go program can vary in length from 3 lines to millions of lines and it should be written into one or more text files with the extension ".go". For example, hello.go. You can use "vi", "vim" or any other text editor to write your Go program into a file. CompilingandExecutingGoPrograms For most of the examples given in this tutorial, you will find a Try it option, so just make use of it and enjoy your learning. Try the following example using the Try it option available at the top right corner of the following sample code: package main import "fmt" func main() { fmt.Println("Hello, World!") }
- 10. Go Programming 3 TryitOptionOnline You really do not need to set up your own environment to start learning Go programming language. Reason is very simple, we already have set up Go Programming environment online, so that you can compile and execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online. Try the following example using the Try it option available at the top right corner of the following sample code displayed on our website: package main import "fmt" func main() { fmt.Println("Hello, World!") } For most of the examples given in this tutorial, you will find a Try it option. LocalEnvironmentSetup If you are still willing to set up your environment for Go programming language, you need the following two software available on your computer: A text editor Go compiler TextEditor You will require a text editor to type your programs. Examples of text editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi. The name and version of text editors can vary on different operating systems. For example, Notepad is used on Windows, and vim or vi is used on Windows as well as Linux or UNIX. 2. ENVIRONMENT SETUP
- 11. Go Programming 4 The files you create with the text editor are called source files. They contain program source code. The source files for Go programs are typically named with the extension ".go". Before starting your programming, make sure you have a text editor in place and you have enough experience to write a computer program, save it in a file, compile it, and finally execute it. TheGoCompiler The source code written in source file is the human readable source for your program. It needs to be compiled and turned into machine language so that your CPU can actually execute the program as per the instructions given. The Go programming language compiler compiles the source code into its final executable program. Go distribution comes as a binary installable for FreeBSD (release 8 and above), Linux, Mac OS X (Snow Leopard and above), and Windows operating systems with 32-bit (386) and 64-bit (amd64) x86 processor architectures. The following section explains how to install Go binary distribution on various OS. DownloadGoArchive Download the latest version of Go installable archive file from Go Downloads. The following version is used in this tutorial: go1.4.windows-amd64.msi. It is copied it into C:>go folder. OS Archive name Windows go1.4.windows-amd64.msi Linux go1.4.linux-amd64.tar.gz Mac go1.4.darwin-amd64-osx10.8.pkg FreeBSD go1.4.freebsd-amd64.tar.gz InstallationonUNIX/Linux/MacOSX,andFreeBSD Extract the download archive into the folder /usr/local, creating a Go tree in /usr/local/go. For example: tar -C /usr/local -xzf go1.4.linux-amd64.tar.gz Add /usr/local/go/bin to the PATH environment variable.
- 12. Go Programming 5 OS Output Linux export PATH=$PATH:/usr/local/go/bin Mac export PATH=$PATH:/usr/local/go/bin FreeBSD export PATH=$PATH:/usr/local/go/bin InstallationonWindows Use the MSI file and follow the prompts to install the Go tools. By default, the installer uses the Go distribution in c:Go. The installer should set the c:Gobin directory in Window's PATH environment variable. Restart any open command prompts for the change to take effect. VerifyingtheInstallation Create a go file named test.go in C:>Go_WorkSpace. File: test.go package main import "fmt" func main() { fmt.Println("Hello, World!") } Now run test.go to see the result: C:Go_WorkSpace>go run test.go Output Hello, World!
- 13. Go Programming 6 Before we study the basic building blocks of Go programming language, let us first discuss the bare minimum structure of Go programs so that we can take it as a reference in subsequent chapters. HelloWorldExample A Go program basically consists of the following parts: Package Declaration Import Packages Functions Variables Statements and Expressions Comments Let us look at a simple code that would print the words "Hello World!": package main import "fmt" func main() { /* This is my first sample program. */ fmt.Println("Hello, World!") } Let us take a look at the various parts of the above program: 1. The first line of the program package main defines the package name in which this program should lie. It is a mandatory statement, as Go programs run in packages. The main package is the starting point to run the program. Each package has a path and name associated with it. 2. The next line import "fmt" is a preprocessor command which tells the Go compiler to include the files lying in the package fmt. 3. The next line func main() is the main function where the program execution begins. 3. PROGRAM STRUCTURE
- 14. Go Programming 7 4. The next line /*...*/ is ignored by the compiler and it is there to add comments in the program. Comments are also represented using // similar to Java or C++ comments. 5. The next line fmt.Println(...) is another function available in Go which causes the message "Hello, World!" to be displayed on the screen. Here fmt package has exported Println method which is used to display the message on the screen. 6. Notice the capital P of Println method. In Go language, a name is exported if it starts with capital letter. Exported means the function or variable/constant is accessible to the importer of the respective package. ExecutingaGoProgram Let us discuss how to save the source code in a file, compile it, and finally execute the program. Please follow the steps given below: 1. Open a text editor and add the above-mentioned code. 2. Save the file as hello.go 3. Open the command prompt. 4. Go to the directory where you saved the file. 5. Type go run hello.go and press enter to run your code. 6. If there are no errors in your code, then you will see "Hello World!" printed on the screen. $ go run hello.go Hello, World! Make sure the Go compiler is in your path and that you are running it in the directory containing the source file hello.go.
- 15. Go Programming 8 We discussed the basic structure of a Go program in the previous chapter. Now it will be easy to understand the other basic building blocks of the Go programming language. TokensinGo A Go program consists of various tokens. A token is either a keyword, an identifier, a constant, a string literal, or a symbol. For example, the following Go statement consists of six tokens: fmt.Println("Hello, World!") The individual tokens are: fmt . Println ( "Hello, World!" ) LineSeparator In a Go program, the line separator key is a statement terminator. That is, individual statements don't need a special separator like “;” in C. The Go compiler internally places “;” as the statement terminator to indicate the end of one logical entity. For example, take a look at the following statements: fmt.Println("Hello, World!") fmt.Println("I am in Go Programming World!") Comments Comments are like helping texts in your Go program and they are ignored by the compiler. They start with /* and terminate with the characters */ as shown below: /* my first program in Go */ 4. BASIC SYNTAX
- 16. Go Programming 9 You cannot have comments within comments and they do not occur within a string or character literals. Identifiers A Go identifier is a name used to identify a variable, function, or any other user- defined item. An identifier starts with a letter A to Z or a to z or an underscore _ followed by zero or more letters, underscores, and digits (0 to 9). identifier = letter { letter | unicode_digit } . Go does not allow punctuation characters such as @, $, and % within identifiers. Go is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in Go. Here are some examples of acceptable identifiers: mahesh kumar abc move_name a_123 myname50 _temp j a23b9 retVal Keywords The following list shows the reserved words in Go. These reserved words may not be used as constant or variable or any other identifier names. break Default Func interface Select case Defer Go map Struct chan Else Goto package Switch const fallthrough if range Type continue For import return Var WhitespaceinGo Whitespace is the term used in Go to describe blanks, tabs, newline characters, and comments. A line containing only whitespace, possibly with a comment, is known as a blank line, and a Go compiler totally ignores it. Whitespaces separate one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Therefore, in the following statement: var age int;
- 17. Go Programming 10 There must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the following statement: fruit = apples + oranges; // get the total fruit No whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.
- 18. Go Programming 11 In the Go programming language, data types refer to an extensive system used for declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted. The types in Go can be classified as follows: Sr. No. Types and Description 1 Boolean types They are boolean types and consists of the two predefined constants: (a) true (b) false 2 Numeric types They are again arithmetic types and they represents a) integer types or b) floating point values throughout the program. 3 String types A string type represents the set of string values. Its value is a sequence of bytes. Strings are immutable types. That is, once they are created, it is not possible to change the contents of a string. The predeclared string type is string. 4 Derived types They include (a) Pointer types, (b) Array types, (c) Structure types, (d) Union types and (e) Function types f) Slice types g) Function types h) Interface types i) Map types j) Channel Types Array types and structure types are collectively referred to as aggregate types. The type of a function specifies the set of all functions with the same parameter and result types. We will discuss the basic types in the following section, whereas other types will be covered in the upcoming chapters. 5. DATA TYPES
- 19. Go Programming 12 IntegerTypes The predefined architecture-independent integer types are: Sr. No. Types and Description 1 uint8 Unsigned 8-bit integers (0 to 255) 2 uint16 Unsigned 16-bit integers (0 to 65535) 3 uint32 Unsigned 32-bit integers (0 to 4294967295) 4 uint64 Unsigned 64-bit integers (0 to 18446744073709551615) 5 int8 Signed 8-bit integers (-128 to 127) 6 int16 Signed 16-bit integers (-32768 to 32767) 7 int32 Signed 32-bit integers (-2147483648 to 2147483647) 8 int64 Signed 64-bit integers (-9223372036854775808 to 9223372036854775807) FloatingTypes The predefined architecture-independent float types are: Sr. No. Types and Description 1 float32 IEEE-754 32-bit floating-point numbers 2 float64 IEEE-754 64-bit floating-point numbers
- 20. Go Programming 13 3 complex64 Complex numbers with float32 real and imaginary parts 4 complex128 Complex numbers with float64 real and imaginary parts The value of an n-bit integer is n bits and is represented using two's complement arithmetic operations. OtherNumericTypes There is also a set of numeric types with implementation-specific sizes: Sr. No. Types and Description 1 byte same as uint8 2 rune same as int32 3 uint 32 or 64 bits 4 int same size as uint 5 uintptr an unsigned integer to store the uninterpreted bits of a pointer value
- 21. Go Programming 14 A variable is nothing but a name given to a storage area that the programs can manipulate. Each variable in Go has a specific type, which determines the size and layout of the variable's memory, the range of values that can be stored within that memory, and the set of operations that can be applied to the variable. The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because Go is case-sensitive. Based on the basic types explained in the previous chapter, there will be the following basic variable types: Type Description byte Typically a single octet(one byte). This is an byte type. int The most natural size of integer for the machine. float32 A single-precision floating point value. Go programming language also allows to define various other types of variables such as Enumeration, Pointer, Array, Structure, and Union, which we will discuss in subsequent chapters. In this chapter, we will focus only basic variable types. VariableDefinitioninGo A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows: var variable_list optional_data_type; Here, optional_data_type is a valid Go data type including byte, int, float32, complex64, boolean or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here: var i, j, k int; var c, ch byte; var f, salary float32; d = 42; 6. VARIABLES
- 22. Go Programming 15 The statement “var i, j, k;” declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j, and k of type int. Variables can be initialized (assigned an initial value) in their declaration. The type of variable is automatically judged by the compiler based on the value passed to it. The initializer consists of an equal sign followed by a constant expression as follows: variable_name = value; For example, d = 3, f = 5; // declaration of d and f. Here d and f are int For definition without an initializer: variables with static storage duration are implicitly initialized with nil (all bytes have the value 0); the initial value of all other variables is zero value of their data type. StaticTypeDeclarationinGo A static type variable declaration provides assurance to the compiler that there is one variable available with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail of the variable. A variable declaration has its meaning at the time of compilation only, the compiler needs the actual variable declaration at the time of linking of the program. Example Try the following example, where the variable has been declared with a type and initialized inside the main function: package main import "fmt" func main() { var x float64 x = 20.0 fmt.Println(x) fmt.Printf("x is of type %Tn", x) } When the above code is compiled and executed, it produces the following result:
- 23. Go Programming 16 20 x is of type float64 DynamicTypeDeclaration/TypeInferenceinGo A dynamic type variable declaration requires the compiler to interpret the type of the variable based on the value passed to it. The compiler does not require a variable to have type statically as a necessary requirement. Example Try the following example, where the variables have been declared without any type. Notice, in case of type inference, we initialized the variable y with := operator, whereas x is initialized using = operator. package main import "fmt" func main() { var x float64 = 20.0 y := 42 fmt.Println(x) fmt.Println(y) fmt.Printf("x is of type %Tn", x) fmt.Printf("y is of type %Tn", y) } When the above code is compiled and executed, it produces the following result: 20 42 x is of type float64 y is of type int MixedVariableDeclarationinGo Variables of different types can be declared in one go using type inference.
- 24. Go Programming 17 Example package main import "fmt" func main() { var a, b, c = 3, 4, "foo" fmt.Println(a) fmt.Println(b) fmt.Println(c) fmt.Printf("a is of type %Tn", a) fmt.Printf("b is of type %Tn", b) fmt.Printf("c is of type %Tn", c) } When the above code is compiled and executed, it produces the following result: 3 4 foo a is of type int b is of type int c is of type string ThelvaluesandthervaluesinGo There are two kinds of expressions in Go: 1. lvalue: Expressions that refer to a memory location is called "lvalue" expression. An lvalue may appear as either the left-hand or right-hand side of an assignment. 2. rvalue: The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right- but not left- hand side of an assignment.
- 25. Go Programming 18 Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and cannot appear on the left-hand side. The following statement is valid: x = 20.0 The following statement is not valid. It would generate compile-time error: 10 = 20
- 26. Go Programming 19 Constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals. Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are also enumeration constants as well. Constants are treated just like regular variables except that their values cannot be modified after their definition. IntegerLiterals An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal. An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order. Here are some examples of integer literals: 212 /* Legal */ 215u /* Legal */ 0xFeeL /* Legal */ 078 /* Illegal: 8 is not an octal digit */ 032UU /* Illegal: cannot repeat a suffix */ Following are other examples of various type of Integer literals: 85 /* decimal */ 0213 /* octal */ 0x4b /* hexadecimal */ 30 /* int */ 30u /* unsigned int */ 30l /* long */ 30ul /* unsigned long */ 7. CONSTANTS
- 27. Go Programming 20 Floating-pointLiterals A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form. While representing using decimal form, you must include the decimal point, the exponent, or both and while representing using exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E. Here are some examples of floating-point literals: 3.14159 /* Legal */ 314159E-5L /* Legal */ 510E /* Illegal: incomplete exponent */ 210f /* Illegal: no decimal or exponent */ .e55 /* Illegal: missing integer or fraction */ EscapeSequence When certain characters are preceded by a backslash, they will have a special meaning in Go. These are known as Escape Sequence codes which are used to represent newline (n), tab (t), backspace, etc. Here, you have a list of some of such escape sequence codes: Escape sequence Meaning character ' ' character " " character ? ? character a Alert or bell b Backspace f Form feed n Newline r Carriage return
- 28. Go Programming 21 t Horizontal tab v Vertical tab ooo Octal number of one to three digits xhh . . . Hexadecimal number of one or more digits The following example shows how to use t in a program: package main import "fmt" func main() { fmt.Printf("HellotWorld!") } When the above code is compiled and executed, it produces the following result: Hello World! StringLiteralsinGo String literals or constants are enclosed in double quotes "". A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters. You can break a long line into multiple lines using string literals and separating them using whitespaces. Here are some examples of string literals. All the three forms are identical strings. "hello, dear" "hello, dear" "hello, " "d" "ear"
- 29. Go Programming 22 TheconstKeyword You can use const prefix to declare constants with a specific type as follows: const variable type = value; The following example shows how to use the const keyword: package main import "fmt" func main() { const LENGTH int = 10 const WIDTH int = 5 var area int area = LENGTH * WIDTH fmt.Printf("value of area : %d", area) } When the above code is compiled and executed, it produces the following result: value of area : 50 Note that it is a good programming practice to define constants in CAPITALS.
- 30. Go Programming 23 An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. Go language is rich in built-in operators and provides the following types of operators: Arithmetic Operators Relational Operators Logical Operators Bitwise Operators Assignment Operators Miscellaneous Operators This tutorial explains arithmetic, relational, logical, bitwise, assignment, and other operators one by one. ArithmeticOperators Following table shows all the arithmetic operators supported by Go language. Assume variable A holds 10 and variable B holds 20 then: Operator Description Example + Adds two operands A + B gives 30 - Subtracts second operand from the first A - B gives -10 * Multiplies both operands A * B gives 200 / Divides the numerator by the denominator. B / A gives 2 % Modulus operator; gives the remainder after an integer division. B % A gives 0 ++ Increment operator. It increases the integer value by one. A++ gives 11 -- Decrement operator. It decreases the integer value by one. A-- gives 9 8. OPERATORS
- 31. Go Programming 24 Example Try the following example to understand all the arithmetic operators available in Go programming language: package main import "fmt" func main() { var a int = 21 var b int = 10 var c int c = a + b fmt.Printf("Line 1 - Value of c is %dn", c ) c = a - b fmt.Printf("Line 2 - Value of c is %dn", c ) c = a * b fmt.Printf("Line 3 - Value of c is %dn", c ) c = a / b fmt.Printf("Line 4 - Value of c is %dn", c ) c = a % b fmt.Printf("Line 5 - Value of c is %dn", c ) a++ fmt.Printf("Line 6 - Value of a is %dn", a ) a-- fmt.Printf("Line 7 - Value of a is %dn", a ) } When you compile and execute the above program, it produces the following result: Line 1 - Value of c is 31 Line 2 - Value of c is 11 Line 3 - Value of c is 210 Line 4 - Value of c is 2
- 32. Go Programming 25 Line 5 - Value of c is 1 Line 6 - Value of a is 22 Line 7 - Value of a is 21 RelationalOperators The following table lists all the relational operators supported by Go language. Assume variable A holds 10 and variable B holds 20, then: Operator Description Example == It checks if the values of two operands are equal or not; if yes, the condition becomes true. (A == B) is not true. != It checks if the values of two operands are equal or not; if the values are not equal, then the condition becomes true. (A != B) is true. > It checks if the value of left operand is greater than the value of right operand; if yes, the condition becomes true. (A > B) is not true. < It checks if the value of left operand is less than the value of the right operand; if yes, the condition becomes true. (A < B) is true. >= It checks if the value of the left operand is greater than or equal to the value of the right operand; if yes, the condition becomes true. (A >= B) is not true. <= It checks if the value of left operand is less than or equal to the value of right operand; if yes, the condition becomes true. (A <= B) is true. Example Try the following example to understand all the relational operators available in Go programming language: package main import "fmt"
- 33. Go Programming 26 func main() { var a int = 21 var b int = 10 if( a == b ) { fmt.Printf("Line 1 - a is equal to bn" ) } else { fmt.Printf("Line 1 - a is not equal to bn" ) } if ( a < b ) { fmt.Printf("Line 2 - a is less than bn" ) } else { fmt.Printf("Line 2 - a is not less than bn" ) } if ( a > b ) { fmt.Printf("Line 3 - a is greater than bn" ) } else { fmt.Printf("Line 3 - a is not greater than bn" ) } /* Lets change value of a and b */ a = 5 b = 20 if ( a <= b ) { fmt.Printf("Line 4 - a is either less than or equal to bn" ) } if ( b >= a ) { fmt.Printf("Line 5 - b is either greater than or equal to bn" ) } } When you compile and execute the above program, it produces the following result: Line 1 - a is not equal to b Line 2 - a is not less than b
- 34. Go Programming 27 Line 3 - a is greater than b Line 4 - a is either less than or equal to b Line 5 - b is either greater than or equal to b LogicalOperators The following table lists all the logical operators supported by Go language. Assume variable A holds 1 and variable B holds 0, then: Operator Description Example && Called Logical AND operator. If both the operands are non-zero, then condition becomes true. (A && B) is false. || Called Logical OR Operator. If any of the two operands is non-zero, then condition becomes true. (A || B) is true. ! Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true, then Logical NOT operator will make false. !(A && B) is true. The following table shows all the logical operators supported by Go language. Assume variable A holds true and variable B holds false, then: Operator Description Example && Called Logical AND operator. If both the operands are false, then the condition becomes false. (A && B) is false. || Called Logical OR Operator. If any of the two operands is true, then the condition becomes true. (A || B) is true. ! Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true, then Logical NOT operator will make it false. !(A && B) is true.
- 35. Go Programming 28 Example Try the following example to understand all the logical operators available in Go programming language: package main import "fmt" func main() { var a bool = true var b bool = false if ( a && b ) { fmt.Printf("Line 1 - Condition is truen" ) } if ( a || b ) { fmt.Printf("Line 2 - Condition is truen" ) } /* lets change the value of a and b */ a = false b = true if ( a && b ) { fmt.Printf("Line 3 - Condition is truen" ) } else { fmt.Printf("Line 3 - Condition is not truen" ) } if ( !(a && b) ) { fmt.Printf("Line 4 - Condition is truen" ) } } When you compile and execute the above program, it produces the following result: Line 2 - Condition is true Line 3 - Condition is not true Line 4 - Condition is true
- 36. Go Programming 29 BitwiseOperators Bitwise operators work on bits and perform bit-by-bit operation. The truth tables for &, |, and ^ are as follows: p q p & q p | q p ^ q 0 0 0 0 0 0 1 0 1 1 1 1 1 1 0 1 0 0 1 1 Assume A = 60; and B = 13. In binary format, they will be as follows: A = 0011 1100 B = 0000 1101 ----------------- A&B = 0000 1100 A|B = 0011 1101 A^B = 0011 0001 ~A = 1100 0011 The Bitwise operators supported by C language are listed in the following table. Assume variable A holds 60 and variable B holds 13, then: Operator Description Example & Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12, which is 0000 1100 | Binary OR Operator copies a bit if it exists in either operand. (A | B) will give 61, which is 0011 1101 ^ Binary XOR Operator copies the bit if it is set in one operand but not both. (A ^ B) will give 49, which is 0011 0001 << Binary Left Shift Operator. The left operands value is moved left by the A << 2 will give 240 which is 1111 0000
- 37. Go Programming 30 number of bits specified by the right operand. >> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. A >> 2 will give 15 which is 0000 1111 Example Try the following example to understand all the bitwise operators available in Go programming language: package main import "fmt" func main() { var a uint = 60 /* 60 = 0011 1100 */ var b uint = 13 /* 13 = 0000 1101 */ var c uint = 0 c = a & b /* 12 = 0000 1100 */ fmt.Printf("Line 1 - Value of c is %dn", c ) c = a | b /* 61 = 0011 1101 */ fmt.Printf("Line 2 - Value of c is %dn", c ) c = a ^ b /* 49 = 0011 0001 */ fmt.Printf("Line 3 - Value of c is %dn", c ) c = a << 2 /* 240 = 1111 0000 */ fmt.Printf("Line 4 - Value of c is %dn", c ) c = a >> 2 /* 15 = 0000 1111 */ fmt.Printf("Line 5 - Value of c is %dn", c ) }
- 38. Go Programming 31 When you compile and execute the above program, it produces the following result: Line 1 - Value of c is 12 Line 2 - Value of c is 61 Line 3 - Value of c is 49 Line 4 - Value of c is 240 Line 5 - Value of c is 15 AssignmentOperators The following table lists all the assignment operators supported by Go language: Operator Description Example = Simple assignment operator, Assigns values from right side operands to left side operand C = A + B assigns the value of A + B into C += Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand C += A is equivalent to C = C + A -= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand C -= A is equivalent to C = C - A *= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand C *= A is equivalent to C = C * A /= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand C /= A is equivalent to C = C / A %= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand C %= A is equivalent to C = C % A
- 39. Go Programming 32 <<= Left shift AND assignment operator C <<= 2 is same as C = C << 2 >>= Right shift AND assignment operator C >>= 2 is same as C = C >> 2 &= Bitwise AND assignment operator C &= 2 is same as C = C & 2 ^= bitwise exclusive OR and assignment operator C ^= 2 is same as C = C ^ 2 |= bitwise inclusive OR and assignment operator C |= 2 is same as C = C | 2 Example Try the following example to understand all the assignment operators available in Go programming language: package main import "fmt" func main() { var a int = 21 var c int c = a fmt.Printf("Line 1 - = Operator Example, Value of c = %dn", c ) c += a fmt.Printf("Line 2 - += Operator Example, Value of c = %dn", c ) c -= a fmt.Printf("Line 3 - -= Operator Example, Value of c = %dn", c ) c *= a fmt.Printf("Line 4 - *= Operator Example, Value of c = %dn", c )
- 40. Go Programming 33 c /= a fmt.Printf("Line 5 - /= Operator Example, Value of c = %dn", c ) c = 200; c <<= 2 fmt.Printf("Line 6 - <<= Operator Example, Value of c = %dn", c ) c >>= 2 fmt.Printf("Line 7 - >>= Operator Example, Value of c = %dn", c ) c &= 2 fmt.Printf("Line 8 - &= Operator Example, Value of c = %dn", c ) c ^= 2 fmt.Printf("Line 9 - ^= Operator Example, Value of c = %dn", c ) c |= 2 fmt.Printf("Line 10 - |= Operator Example, Value of c = %dn", c ) } When you compile and execute the above program, it produces the following result: Line 1 - = Operator Example, Value of c = 21 Line 2 - += Operator Example, Value of c = 42 Line 3 - -= Operator Example, Value of c = 21 Line 4 - *= Operator Example, Value of c = 441 Line 5 - /= Operator Example, Value of c = 21 Line 6 - <<= Operator Example, Value of c = 800 Line 7 - >>= Operator Example, Value of c = 200 Line 8 - &= Operator Example, Value of c = 0 Line 9 - ^= Operator Example, Value of c = 2 Line 10 - |= Operator Example, Value of c = 2
- 41. Go Programming 34 MiscellaneousOperators There are a few other important operators supported by Go Language including sizeof and ?:. Operator Description Example & Returns the address of a variable. &a; provides actual address of the variable. * Pointer to a variable. *a; provides pointer to a variable. Example Try following example to understand all the miscellaneous operators available in Go programming language: package main import "fmt" func main() { var a int = 4 var b int32 var c float32 var ptr *int /* example of type operator */ fmt.Printf("Line 1 - Type of variable a = %Tn", a ); fmt.Printf("Line 2 - Type of variable b = %Tn", b ); fmt.Printf("Line 3 - Type of variable c= %Tn", c ); /* example of & and * operators */ ptr = &a /* 'ptr' now contains the address of 'a'*/ fmt.Printf("value of a is %dn", a); fmt.Printf("*ptr is %d.n", *ptr); } When you compile and execute the above program, it produces the following result:
- 42. Go Programming 35 Line 1 - Type of variable a = int Line 2 - Type of variable b = int32 Line 3 - Type of variable c= float32 value of a is 4 *ptr is 4. OperatorsPrecedenceinGo Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator. For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7. Here, operators with the highest precedence appear at the top of the table, and those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first. Category Operator Associativity Postfix () [] -> . ++ - - Left to right Unary + - ! ~ ++ - - (type)* & sizeof Right to left Multiplicative * / % Left to right Additive + - Left to right Shift << >> Left to right Relational < <= > >= Left to right Equality == != Left to right Bitwise AND & Left to right Bitwise XOR ^ Left to right Bitwise OR | Left to right Logical AND && Left to right
- 43. Go Programming 36 Logical OR || Left to right Conditional ?: Right to left Assignment = += -= *= /= %=>>= <<= &= ^= |= Right to left Comma , Left to right Example Try the following example to understand the operator precedence available in Go programming language: package main import "fmt" func main() { var a int = 20 var b int = 10 var c int = 15 var d int = 5 var e int; e = (a + b) * c / d; // ( 30 * 15 ) / 5 fmt.Printf("Value of (a + b) * c / d is : %dn", e ); e = ((a + b) * c) / d; // (30 * 15 ) / 5 fmt.Printf("Value of ((a + b) * c) / d is : %dn" , e ); e = (a + b) * (c / d); // (30) * (15/5) fmt.Printf("Value of (a + b) * (c / d) is : %dn", e ); e = a + (b * c) / d; // 20 + (150/5) fmt.Printf("Value of a + (b * c) / d is : %dn" , e ); }
- 44. Go Programming 37 When you compile and execute the above program, it produces the following result: Value of (a + b) * c / d is : 90 Value of ((a + b) * c) / d is : 90 Value of (a + b) * (c / d) is : 90 Value of a + (b * c) / d is : 50
- 45. Go Programming 38 Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false. Following is the general form of a typical decision making structure found in most of the programming languages: Go programming language provides the following types of decision making statements. Click the following links to check their detail. Statement Description if statement An if statement consists of a boolean expression followed by one or more statements. if...else statement An if statement can be followed by an optional else statement, which executes when the boolean expression is false. nested if statements You can use one if or else if statement inside another if or else if statement(s). 9. DECISION MAKING
- 46. Go Programming 39 switch statement A switch statement allows a variable to be tested for equality against a list of values. select statement A select statement is similar to switch statement with difference that case statements refers to channel communications. TheifStatement An if statement consists of a boolean expression followed by one or more statements. Syntax The syntax of an if statement in Go programming language is: if(boolean_expression) { /* statement(s) will execute if the boolean expression is true */ } If the boolean expression evaluates to true, then the block of code inside the if statement is executed. If boolean expression evaluates to false, then the first set of code after the end of the if statement (after the closing curly brace) is executed. Flow Diagram
- 47. Go Programming 40 Example package main import "fmt" func main() { /* local variable definition */ var a int = 10 /* check the boolean condition using if statement */ if( a < 20 ) { /* if condition is true then print the following */ fmt.Printf("a is less than 20n" ) } fmt.Printf("value of a is : %dn", a) } When the above code is compiled and executed, it produces the following result: a is less than 20; value of a is : 10 Theif…elseStatement An if statement can be followed by an optional else statement, which executes when the boolean expression is false. Syntax The syntax of an if...else statement in Go programming language is: if(boolean_expression) { /* statement(s) will execute if the boolean expression is true */ } else { /* statement(s) will execute if the boolean expression is false */
- 48. Go Programming 41 } If the boolean expression evaluates to true, then the if block of code is executed, otherwise else block of code is executed. Flow Diagram Example package main import "fmt" func main() { /* local variable definition */ var a int = 100; /* check the boolean condition */ if( a < 20 ) { /* if condition is true then print the following */ fmt.Printf("a is less than 20n" );
- 49. Go Programming 42 } else { /* if condition is false then print the following */ fmt.Printf("a is not less than 20n" ); } fmt.Printf("value of a is : %dn", a); } When the above code is compiled and executed, it produces the following result: a is not less than 20; value of a is : 100 NestedifStatement It is always legal in Go programming to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s). Syntax The syntax for a nested if statement is as follows: if( boolean_expression 1) { /* Executes when the boolean expression 1 is true */ if(boolean_expression 2) { /* Executes when the boolean expression 2 is true */ } } You can nest else if...else in the similar way as you have nested if statement. Example package main import "fmt" func main() {
- 50. Go Programming 43 /* local variable definition */ var a int = 100 var b int = 200 /* check the boolean condition */ if( a == 100 ) { /* if condition is true then check the following */ if( b == 200 ) { /* if condition is true then print the following */ fmt.Printf("Value of a is 100 and b is 200n" ); } } fmt.Printf("Exact value of a is : %dn", a ); fmt.Printf("Exact value of b is : %dn", b ); } When the above code is compiled and executed, it produces the following result: Value of a is 100 and b is 200 Exact value of a is : 100 Exact value of b is : 200 TheSwitchStatement A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case. In Go programming, switch statements are of two types: Expression Switch - In expression switch, a case contains expressions, which is compared against the value of the switch expression. Type Switch - In type switch, a case contain type which is compared against the type of a specially annotated switch expression.
- 51. Go Programming 44 ExpressionSwitch The syntax for expression switch statement in Go programming is as follows: switch(boolean-expression or integral type){ case boolean-expression or integral type : statement(s); case boolean-expression or integral type : statement(s); /* you can have any number of case statements */ default : /* Optional */ statement(s); } The following rules apply to a switch statement: The expression used in a switch statement must have an integral or boolean expression, or be of a class type in which the class has a single conversion function to an integral or boolean value. If the expression is not passed, then the default value is true. You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon. The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal. When the variable being switched on is equal to a case, the statements following that case will execute. No break is needed in the case statement. A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.
- 52. Go Programming 45 Flow Diagram Example package main import "fmt" func main() { /* local variable definition */ var grade string = "B" var marks int = 90 switch marks { case 90: grade = "A" case 80: grade = "B" case 50,60,70 : grade = "C"
- 53. Go Programming 46 default: grade = "D" } switch { case grade == "A" : fmt.Printf("Excellent!n" ) case grade == "B", grade == "C" : fmt.Printf("Well donen" ) case grade == "D" : fmt.Printf("You passedn" ) case grade == "F": fmt.Printf("Better try againn" ) default: fmt.Printf("Invalid graden" ); } fmt.Printf("Your grade is %sn", grade ); } When the above code is compiled and executed, it produces the following result: Well done Excellent! Your grade is A TypeSwitch The syntax for a type switch statement in Go programming is as follows: switch x.(type){ case type: statement(s); case type: statement(s); /* you can have any number of case statements */ default: /* Optional */ statement(s); }
- 54. Go Programming 47 The following rules apply to a switch statement: The expression used in a switch statement must have an variable of interface{} type. You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon. The type for a case must be the same data type as the variable in the switch, and it must be a valid data type. When the variable being switched on is equal to a case, the statements following that case will execute. No break is needed in the case statement. A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case. Example package main import "fmt" func main() { var x interface{} switch i := x.(type) { case nil: fmt.Printf("type of x :%T",i) case int: fmt.Printf("x is int") case float64: fmt.Printf("x is float64") case func(int) float64: fmt.Printf("x is func(int)") case bool, string: fmt.Printf("x is bool or string") default: fmt.Printf("don't know the type") }
- 55. Go Programming 48 } When the above code is compiled and executed, it produces the following result: type of x :<nil> TheSelectStatement The syntax for a select statement in Go programming language is as follows: select { case communication clause : statement(s); case communication clause : statement(s); /* you can have any number of case statements */ default : /* Optional */ statement(s); } The following rules apply to a select statement: You can have any number of case statements within a select. Each case is followed by the value to be compared to and a colon. The type for a case must be the communication channel operation. When the channel operation occurs, the statements following that case is executed. No break is needed in the case statement. A select statement can have an optional default case, which must appear at the end of the select. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case. Example package main import "fmt" func main() { var c1, c2, c3 chan int var i1, i2 int select {
- 56. Go Programming 49 case i1 = <-c1: fmt.Printf("received ", i1, " from c1n") case c2 <- i2: fmt.Printf("sent ", i2, " to c2n") case i3, ok := (<-c3): // same as: i3, ok := <-c3 if ok { fmt.Printf("received ", i3, " from c3n") } else { fmt.Printf("c3 is closedn") } default: fmt.Printf("no communicationn") } } When the above code is compiled and executed, it produces the following result: no communication Theif...elseif...elseStatement An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement. While using if…else if…else statements, there are a few points to keep in mind: An if can have zero or one else's and it must come after any else if's. An if can have zero to many else if's and they must come before the else. Once an else if succeeds, none of the remaining else if's or else's will be tested. Syntax The syntax of if...else if...else statement in Go programming language is: if(boolean_expression 1) { /* Executes when the boolean expression 1 is true */ } else if( boolean_expression 2) { /* Executes when the boolean expression 2 is true */
- 57. Go Programming 50 } else if( boolean_expression 3) { /* Executes when the boolean expression 3 is true */ } else { /* executes when the none of the above condition is true */ } Example package main import "fmt" func main() { /* local variable definition */ var a int = 100 /* check the boolean condition */ if( a == 10 ) { /* if condition is true then print the following */ fmt.Printf("Value of a is 10n" ) } else if( a == 20 ) { /* if else if condition is true */ fmt.Printf("Value of a is 20n" ) } else if( a == 30 ) { /* if else if condition is true */ fmt.Printf("Value of a is 30n" ) } else { /* if none of the conditions is true */ fmt.Printf("None of the values is matchingn" ) } fmt.Printf("Exact value of a is: %dn", a )
- 58. Go Programming 51 } When the above code is compiled and executed, it produces the following result: None of the values is matching Exact value of a is: 100
- 59. Go Programming 52 There may be a situation when you need to execute a block of code several number of times. In general, statements are executed sequentially: the first statement in a function is executed first, followed by the second, and so on. Programming languages provide various control structures that allow more complicated execution paths. A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages: Go programming language provides the following types of loops to handle looping requirements. Loop Type Description for loop It executes a sequence of statements multiple times and abbreviates the code that manages the loop variable. nested loops These are one or multiple loops inside any for loop. forLoop A for loop is a repetition control structure. It allows you to write a loop that needs to execute a specific number of times. 10.LOOPS
- 60. Go Programming 53 Syntax The syntax of for loop in Go programming language is: for [condition | ( init; condition; increment ) | Range] { statement(s); } The flow of control in a for loop is as follows: If a condition is available, then for loop executes as long as condition is true. If a for clause that is ( init; condition; increment ) is present, then: o The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears. o Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and the flow of control jumps to the next statement just after the for loop. o After the body of the for loop executes, the flow of control jumps back up to the increment statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition. o The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again the condition). After the condition becomes false, the for loop terminates. If range is available, then the for loop executes for each item in the range.
- 61. Go Programming 54 Flow Diagram Example package main import "fmt" func main() { var b int = 15 var a int numbers := [6]int{1, 2, 3, 5}
- 62. Go Programming 55 /* for loop execution */ for a := 0; a < 10; a++ { fmt.Printf("value of a: %dn", a) } for a < b { a++ fmt.Printf("value of a: %dn", a) } for i,x:= range numbers { fmt.Printf("value of x = %d at %dn", x,i) } } When the above code is compiled and executed, it produces the following result: value of a: 0 value of a: 1 value of a: 2 value of a: 3 value of a: 4 value of a: 5 value of a: 6 value of a: 7 value of a: 8 value of a: 9 value of a: 1 value of a: 2 value of a: 3 value of a: 4 value of a: 5 value of a: 6 value of a: 7
- 63. Go Programming 56 value of a: 8 value of a: 9 value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of x = 1 at 0 value of x = 2 at 1 value of x = 3 at 2 value of x = 5 at 3 value of x = 0 at 4 value of x = 0 at 5 NestedforLoops Go programming language allows to use one loop inside another loop. The following section shows a few examples to illustrate the concept: Syntax The syntax for a nested for loop statement in Go is as follows: for [condition | ( init; condition; increment ) | Range] { for [condition | ( init; condition; increment ) | Range] { statement(s); } statement(s); }
- 64. Go Programming 57 Example The following program uses a nested for loop to find the prime numbers from 2 to 100: package main import "fmt" func main() { /* local variable definition */ var i, j int for i=2; i < 100; i++ { for j=2; j <= (i/j); j++ { if(i%j==0) { break; // if factor found, not prime } } if(j > (i/j)) { fmt.Printf("%d is primen", i); } } } When the above code is compiled and executed, it produces the following result: 2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime
- 65. Go Programming 58 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 53 is prime 59 is prime 61 is prime 67 is prime 71 is prime 73 is prime 79 is prime 83 is prime 89 is prime 97 is prime LoopControlStatements Loop control statements change an execution from its normal sequence. When an execution leaves its scope, all automatic objects that were created in that scope are destroyed. Go supports the following control statements: Control Statement Description break statement It terminates a for loop or switch statement and transfers execution to the statement immediately following the for loop or switch. continue statement It causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. goto statement It transfers control to the labeled statement. The break statement in Go programming language has the following two usages: 1. When a break statement is encountered inside a loop, the loop is immediately terminated and the program control resumes at the next statement following the loop.
- 66. Go Programming 59 2. It can be used to terminate a case in a switch statement. If you are using nested loops, the break statement will stop the execution of the innermost loop and start executing the next line of code after the block. Syntax The syntax for a break statement in Go is as follows: break; Flow Diagram Example package main import "fmt" func main() { /* local variable definition */ var a int = 10
- 67. Go Programming 60 /* for loop execution */ for a < 20 { fmt.Printf("value of a: %dn", a); a++; if a > 15 { /* terminate the loop using break statement */ break; } } } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 ThecontinueStatement The continue statement in Go programming language works somewhat like a break statement. Instead of forcing termination, a continue statement forces the next iteration of the loop to take place, skipping any code in between. In case of the for loop, continue statement causes the conditional test and increment portions of the loop to execute. Syntax The syntax for a continue statement in Go is as follows: continue;
- 68. Go Programming 61 Flow Diagram Example package main import "fmt" func main() { /* local variable definition */ var a int = 10 /* do loop execution */ for a < 20 { if a == 15 { /* skip the iteration */ a = a + 1; continue;
- 69. Go Programming 62 } fmt.Printf("value of a: %dn", a); a++; } } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19 ThegotoStatement A goto statement in Go programming language provides an unconditional jump from the goto to a labeled statement in the same function. Note: Use of goto statement is highly discouraged in any programming language because it becomes difficult to trace the control flow of a program, making the program difficult to understand and hard to modify. Any program that uses a goto can be rewritten using some other construct. Syntax The syntax for a goto statement in Go is as follows: goto label; .. . label: statement; Here, label can be any plain text except Go keyword and it can be set anywhere in the Go program above or below to goto statement.
- 70. Go Programming 63 Flow Diagram Example package main import "fmt" func main() { /* local variable definition */ var a int = 10 /* do loop execution */ LOOP: for a < 20 { if a == 15 { /* skip the iteration */ a = a + 1
- 71. Go Programming 64 goto LOOP } fmt.Printf("value of a: %dn", a) a++ } } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19 TheInfiniteLoop A loop becomes an infinite loop if its condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty or by passing true to it. package main import "fmt" func main() { for true { fmt.Printf("This loop will run forever.n"); } }
- 72. Go Programming 65 When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C programmers more commonly use the for(;;) construct to signify an infinite loop. Note: You can terminate an infinite loop by pressing Ctrl + C keys.
- 73. Go Programming 66 A function is a group of statements that together perform a task. Every Go program has at least one function, which is main(). You can divide your code into separate functions. How you divide your code among different functions is up to you, but logically, the division should be such that each function performs a specific task. A function declaration tells the compiler about a function name, return type, and parameters. A function definition provides the actual body of the function. The Go standard library provides numerous built-in functions that your program can call. For example, the function len() takes arguments of various types and returns the length of the type. If a string is passed to it, the function returns the length of the string in bytes. If an array is passed to it, the function returns the length of the array. Functions are also known as method, sub-routine, or procedure. DefiningaFunction The general form of a function definition in Go programming language is as follows: func function_name( [parameter list] ) [return_types] { body of the function } A function definition in Go programming language consists of a function header and a function body. Here are all the parts of a function: Func: It starts the declaration of a function. Function Name: It is the actual name of the function. The function name and the parameter list together constitute the function signature. Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters. Return Type: A function may return a list of values. The return_types is the list of data types of the values the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the not required. 11.FUNCTIONS
- 74. Go Programming 67 Function Body: It contains a collection of statements that define what the function does. Example The following source code shows a function called max(). This function takes two parameters num1 and num2 and returns the maximum between the two: /* function returning the max between two numbers */ func max(num1, num2 int) int { /* local variable declaration */ result int if (num1 > num2) { result = num1 } else { result = num2 } return result } CallingaFunction While creating a Go function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task. When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program. To call a function, you simply need to pass the required parameters along with its function name. If the function returns a value, then you can store the returned value. For example: package main import "fmt" func main() {
- 75. Go Programming 68 /* local variable definition */ var a int = 100 var b int = 200 var ret int /* calling a function to get max value */ ret = max(a, b) fmt.Printf( "Max value is : %dn", ret ) } /* function returning the max between two numbers */ func max(num1, num2 int) int { /* local variable declaration */ var result int if (num1 > num2) { result = num1 } else { result = num2 } return result } We have kept the max() function along with the main() function and compiled the source code. While running the final executable, it would produce the following result: Max value is : 200 ReturningMultipleValuesfromFunction A Go function can return multiple values. For example: package main import "fmt"
- 76. Go Programming 69 func swap(x, y string) (string, string) { return y, x } func main() { a, b := swap("Mahesh", "Kumar") fmt.Println(a, b) } When the above code is compiled and executed, it produces the following result: Kumar Mahesh FunctionArguments If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function. The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit. While calling a function, there are two ways that arguments can be passed to a function: Call Type Description Call by value This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. Call by reference This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument. By default, Go uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function. The above program, while calling the max() function, used the same method.
- 77. Go Programming 70 CallbyValue The call by value method of passing arguments to a function copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. By default, Go programming language uses call by value method to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function. Consider the function swap() definition as follows: /* function definition to swap the values */ func swap(int x, int y) int { var temp int temp = x /* save the value of x */ x = y /* put y into x */ y = temp /* put temp into y */ return temp; } Now, let us call the function swap() by passing actual values as in the following example: package main import "fmt" func main() { /* local variable definition */ var a int = 100 var b int = 200 fmt.Printf("Before swap, value of a : %dn", a ) fmt.Printf("Before swap, value of b : %dn", b ) /* calling a function to swap the values */ swap(a, b)
- 78. Go Programming 71 fmt.Printf("After swap, value of a : %dn", a ) fmt.Printf("After swap, value of b : %dn", b ) } func swap(x, y int) int { var temp int temp = x /* save the value of x */ x = y /* put y into x */ y = temp /* put temp into y */ return temp; } Put the above code in a single C file, and then compile and execute it. It will produce the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :100 After swap, value of b :200 It shows that there is no change in the values though they had been changed inside the function. CallbyReference The call by reference method of passing arguments to a function copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the passed argument. To pass the value by reference, argument pointers are passed to the functions just like any other value. Accordingly, you need to declare the function parameters as pointer types as in the following function swap(), which exchanges the values of the two integer variables pointed to by its arguments. /* function definition to swap the values */ func swap(x *int, y *int) { var temp int temp = *x /* save the value at address x */
- 79. Go Programming 72 *x = *y /* put y into x */ *y = temp /* put temp into y */ } To learn more about pointers in Go programming, please go through Go - Pointers. For now, let us call the function swap() by passing values by reference as in the following example: package main import "fmt" func main() { /* local variable definition */ var a int = 100 var b int= 200 fmt.Printf("Before swap, value of a : %dn", a ) fmt.Printf("Before swap, value of b : %dn", b ) /* calling a function to swap the values. * &a indicates pointer to a ie. address of variable a and * &b indicates pointer to b ie. address of variable b. */ swap(&a, &b) fmt.Printf("After swap, value of a : %dn", a ) fmt.Printf("After swap, value of b : %dn", b ) } func swap(x *int, y *int) { var temp int temp = *x /* save the value at address x */ *x = *y /* put y into x */ *y = temp /* put temp into y */ }
- 80. Go Programming 73 Put the above code in a single C file, and then compile and execute it. It produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :200 After swap, value of b :100 It shows that the change has reflected outside the function as well, unlike call by value where the changes do not reflect outside the function. FunctionUsage A function can be used in the following ways: Function Usage Description Function as Value Functions can be created on the fly and can be used as values. Function Closures Functions closures are anonymous functions that can be used in dynamic programming. Method Methods are special functions with a receiver. Go programming language provides the flexibility to create functions on the fly and use them as values. In the following example, we've initialized a variable with a function definition. Purpose of this function variable is just to use inbuilt math.sqrt() function. For example: package main import ( "fmt" "math" ) func main(){ /* declare a function variable */ getSquareRoot := func(x float64) float64 { return math.Sqrt(x)
- 81. Go Programming 74 } /* use the function */ fmt.Println(getSquareRoot(9)) } When the above code is compiled and executed, it produces the following result: 3 FunctionClosures Go programming language supports anonymous functions which can act as function closures. Anonymous functions are used when we want to define a function inline without passing any name to it. In our example, we created a function getSequence() which returns another function. The purpose of this function is to close over a variable i of upper function to form a closure. For example: package main import "fmt" func getSequence() func() int { i:=0 return func() int { i+=1 return i } } func main(){ /* nextNumber is now a function with i as 0 */ nextNumber := getSequence() /* invoke nextNumber to increase i by 1 and return the same */ fmt.Println(nextNumber())
- 82. Go Programming 75 fmt.Println(nextNumber()) fmt.Println(nextNumber()) /* create a new sequence and see the result, i is 0 again*/ nextNumber1 := getSequence() fmt.Println(nextNumber1()) fmt.Println(nextNumber1()) } When the above code is compiled and executed, it produces the following result: 1 2 3 1 2 Method Go programming language supports special types of functions called methods. In method declaration syntax, a "receiver" is present to represent the container of the function. This receiver can be used to call a function using "." operator. For example: Syntax func (variable_name variable_data_type) function_name() [return_type]{ /* function body*/ } package main import ( "fmt" "math" ) /* define a circle */ type Circle strut {
- 83. Go Programming 76 x,y,radius float64 } /* define a method for circle */ func(circle Circle) area() float64 { return math.Pi * circle.radius * circle.radius } func main(){ circle := Circle(x:0, y:0, radius:5) fmt.Printf("Circle area: %f", circle.area()) } When the above code is compiled and executed, it produces the following result: Circle area: 78.539816
- 84. Go Programming 77 A scope in any programming is a region of the program where a defined variable can exist and beyond that the variable cannot be accessed. There are three places where variables can be declared in Go programming language: 1. Inside a function or a block (local variables) 2. Outside of all functions (global variables) 3. In the definition of function parameters (formal parameters) Let us find out what are local and global variables and what are formal parameters. LocalVariables Variables that are declared inside a function or a block are called local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. The following example uses local variables. Here all the variables a, b, and c are local to the main() function. package main import "fmt" func main() { /* local variable declaration */ var a, b, c int /* actual initialization */ a = 10 b = 20 c = a + b fmt.Printf ("value of a = %d, b = %d and c = %dn", a, b, c) } When the above code is compiled and executed, it produces the following result: value of a = 10, b = 20 and c = 30 12.SCOPE RULES
- 85. Go Programming 78 GlobalVariables Global variables are defined outside of a function, usually on top of the program. Global variables hold their value throughout the lifetime of the program and they can be accessed inside any of the functions defined for the program. A global variable can be accessed by any function. That is, a global variable is available for use throughout the program after its declaration. The following example uses both global and local variables: package main import "fmt" /* global variable declaration */ var g int func main() { /* local variable declaration */ var a, b int /* actual initialization */ a = 10 b = 20 g = a + b fmt.Printf("value of a = %d, b = %d and g = %dn", a, b, g) } When the above code is compiled and executed, it produces the following result: value of a = 10, b = 20 and c = 30 A program can have the same name for local and global variables but the value of the local variable inside a function takes preference. For example: package main import "fmt"
- 86. Go Programming 79 /* global variable declaration */ var g int = 20 func main() { /* local variable declaration */ var g int = 10 fmt.Printf ("value of g = %dn", g) } When the above code is compiled and executed, it produces the following result: value of g = 10 FormalParameters Formal parameters are treated as local variables with-in that function and they take preference over the global variables. For example: package main import "fmt" /* global variable declaration */ var a int = 20; func main() { /* local variable declaration in main function */ var a int = 10 var b int = 20 var c int = 0 fmt.Printf("value of a in main() = %dn", a); c = sum( a, b); fmt.Printf("value of c in main() = %dn", c); }
- 87. Go Programming 80 /* function to add two integers */ func sum(a, b int) int { fmt.Printf("value of a in sum() = %dn", a); fmt.Printf("value of b in sum() = %dn", b); return a + b; } When the above code is compiled and executed, it produces the following result: value of a in main() = 10 value of a in sum() = 10 value of b in sum() = 20 value of c in main() = 30 InitializingLocalandGlobalVariables Local and global variables are initialized to their default value, which is 0; while pointers are initialized to nil. Data Type Initial Default Value int 0 float32 0 pointer Nil
- 88. Go Programming 81 Strings, which are widely used in Go programming, are a read-only slice of bytes. In the Go programming language, strings are slices. The Go platform provides various libraries to manipulate strings: unicode regexp strings CreatingStrings The most direct way to create a string is to write: var greeting = "Hello world!" Whenever it encounters a string literal in your code, the compiler creates a string object with its value. In this case, it is "Hello world!'. A string literal holds valid UTF-8 sequences called runes. A string holds arbitrary bytes. package main import "fmt" func main() { var greeting = "Hello world!" fmt.Printf("normal string: ") fmt.Printf("%s", greeting) fmt.Printf("n") fmt.Printf("hex bytes: ") for i := 0; i < len(greeting); i++ { fmt.Printf("%x ", greeting[i]) } fmt.Printf("n") const sampleText = "xbdxb2x3dxbcx20xe2x8cx98" 13.STRINGS
- 89. Go Programming 82 /*q flag escapes unprintable characters, with + flag it escapes non- ascii characters as well to make output unambiguous */ fmt.Printf("quoted string: ") fmt.Printf("%+q", sampleText) fmt.Printf("n") } It would produce the following result: normal string: Hello world! hex bytes: 48 65 6c 6c 6f 20 77 6f 72 6c 64 21 quoted string: "xbdxb2=xbc u2318" Note: The string literal is immutable. Once it is created, a string literal cannot be changed. StringLength len(str) method returns the number of bytes contained in a string literal. package main import "fmt" func main() { var greeting = "Hello world!" fmt.Printf("String Length is: ") fmt.Println(len(greeting)) } It would produce the following result: String Length is : 12 ConcatenatingStrings The strings package includes a method join for concatenating multiple strings: strings.Join(sample, " ") Join concatenates the elements of an array to create a single string. Second parameter is a separator which is placed between the elements of the array.
- 90. Go Programming 83 Let us look at the following example: package main import ( "fmt" "strings" ) func main() { greetings := []string{"Hello","world!"} fmt.Println(strings.Join(greetings, " ")) } It would produce the following result: Hello world!
- 91. Go Programming 84 Go programming language provides a data structure called the array, which can store a fixed-size sequential collection of elements of the same data type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type. Instead of declaring individual variables, such as number0, number1, ..., number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index. All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element. DeclaringArrays To declare an array in Go, a programmer specifies the type of the elements and the number of elements required by an array as follows: var variable_name [SIZE] variable_type This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid Go data type. For example, to declare a 10-element array called balance of type float32, use this statement: var balance [10] float32 Here, balance is a variable array that can hold up to 10 float numbers. InitializingArrays You can initialize an array in Go, either one by one or using a single statement as follows: var balance = [5]float32{1000.0, 2.0, 3.4, 7.0, 50.0} 14.ARRAYS
- 92. Go Programming 85 The number of values between the braces { } cannot be larger than the number of elements that we declare for the array between the square brackets [ ]. If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write: var balance = []float32{1000.0, 2.0, 3.4, 7.0, 50.0} You will create exactly the same array as you did in the previous example. Following is an example to assign a single element of the array: balance[4] = 50.0 The above statement assigns element number 5th in the array with a value of 50.0. All arrays have 0 as the index of their first element which is also called base index and last index of an array will be total size of the array minus 1. Following is the pictorial representation of the same array we discussed above: AccessingArrayElements An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example: float32 salary = balance[9] The above statement will take the 10th element from the array and assign the value to salary variable. Following is an example which will use all the above- mentioned three concepts, viz. declaration, assignment, and accessing arrays: package main import "fmt" func main() { var n [10]int /* n is an array of 10 integers */ var i,j int /* initialize elements of array n to 0 */ for i = 0; i < 10; i++ {
- 93. Go Programming 86 n[i] = i + 100 /* set element at location i to i + 100 */ } /* output each array element's value */ for j = 0; j < 10; j++ { fmt.Printf("Element[%d] = %dn", j, n[j] ) } } When the above code is compiled and executed, it produces the following result: Element[0] = 100 Element[1] = 101 Element[2] = 102 Element[3] = 103 Element[4] = 104 Element[5] = 105 Element[6] = 106 Element[7] = 107 Element[8] = 108 Element[9] = 109 GoArraysinDetail There are important concepts related to array which should be clear to a Go programmer: Concept Description Multi-dimensional arrays Go supports multidimensional arrays. The simplest form of a multidimensional array is the two-dimensional array. Passing arrays to functions You can pass to the function a pointer to an array by specifying the array's name without an index.
- 94. Go Programming 87 MultidimensionalArraysinGo Go programming language allows multidimensional arrays. Here is the general form of a multidimensional array declaration: var variable_name [SIZE1][SIZE2]...[SIZEN] variable_type For example, the following declaration creates a three-dimensional [5][10][4] integer array: var threedim [5][10][4]int Two-DimensionalArrays A two-dimensional array is the simplest form of a multidimensional array. A two- dimensional array is, in essence, a list of one-dimensional arrays. To declare a two-dimensional integer array of size [x, y], you would write something as follows: var arrayName [ x ][ y ] variable_type Where variable_type can be any valid Go data type and arrayName can be a valid Go identifier. A two-dimensional array can be taken as a table having x number of rows and y number of columns. A 2-dimensional array a, which contains three rows and four columns can be shown as below: Thus, every element in the array a is identified by an element name as a[i][j], where a is the name of the array, and i and j are the subscripts that uniquely identify each element in a. InitializingTwo-DimensionalArrays Multidimensional arrays may be initialized by specifying bracketed values for each row. The following array is with 3 rows and each row has 4 columns. a = [3][4]int{ {0, 1, 2, 3} , /* initializers for row indexed by 0 */ {4, 5, 6, 7} , /* initializers for row indexed by 1 */
- 95. Go Programming 88 {8, 9, 10, 11} /* initializers for row indexed by 2 */ } AccessingTwo-DimensionalArrayElements An element in a two-dimensional array is accessed by using the subscripts, i.e., row index and column index of the array. For example: int val = a[2][3] The above statement takes the 4th element from the 3rd row of the array. You can verify it in the above diagram. Let us check the following program where we have used nested loop to handle a two-dimensional array: package main import "fmt" func main() { /* an array with 5 rows and 2 columns*/ var a = [5][2]int{ {0,0}, {1,2}, {2,4}, {3,6},{4,8}} var i, j int /* output each array element's value */ for i = 0; i < 5; i++ { for j = 0; j < 2; j++ { fmt.Printf("a[%d][%d] = %dn", i,j, a[i][j] ) } } } When the above code is compiled and executed, it produces the following result: a[0][0]: 0 a[0][1]: 0 a[1][0]: 1 a[1][1]: 2 a[2][0]: 2 a[2][1]: 4
- 96. Go Programming 89 a[3][0]: 3 a[3][1]: 6 a[4][0]: 4 a[4][1]: 8 As explained above, you can have arrays with any number of dimensions, although it is likely that most of the arrays you create will be of one or two dimensions. PassingArraystoFunctions If you want to pass a one-dimensional array as an argument in a function, you would have to declare a function formal parameter in one of following two ways. Both the declaration methods produce similar results because each tells the compiler that an integer array is going to be received. Similarly, you can pass a multidimensional array as a formal parameter. Way-1 Formal parameters as a sized array as follows: void myFunction(param [10]int) { . . . } Way-2 Formal parameters as an unsized array as follows: void myFunction(param []int) { . . . }
- 97. Go Programming 90 Example Now, consider the following function, which will take an array as an argument along with another argument and based on the passed arguments, it will return the average of the numbers passed through the array as follows: func getAverage(arr []int, int size) float32 { var i int var avg, sum float32 for i = 0; i < size; ++i { sum += arr[i] } avg = sum / size return avg; } Now, let us call the above function as follows: package main import "fmt" func main() { /* an int array with 5 elements */ var balance = []int {1000, 2, 3, 17, 50} var avg float32 /* pass array as an argument */ avg = getAverage( balance, 5 ) ; /* output the returned value */ fmt.Printf( "Average value is: %f ", avg ); } func getAverage(arr []int, size int) float32 {
- 98. Go Programming 91 var i,sum int var avg float32 for i = 0; i < size;i++ { sum += arr[i] } avg = float32(sum / size) return avg; } When the above code is compiled together and executed, it produces the following result: Average value is: 214.400000 As you can see, the length of the array doesn't matter as far as the function is concerned because Go performs no bounds checking for formal parameters.
- 99. Go Programming 92 Pointers in Go are easy and fun to learn. Some Go programming tasks are performed more easily with pointers, and other tasks, such as call by reference, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Go programmer. As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined: package main import "fmt" func main() { var a int = 10 fmt.Printf("Address of a variable: %xn", &a ) } When the above code is compiled and executed, it produces the following result: Address of a variable: 10328000 So you understood what is memory address and how to access it. Now let us see what pointers are. WhatArePointers? A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is: var var_name *var-type Here, type is the pointer's base type; it must be a valid C data type and var- name is the name of the pointer variable. The asterisk * you used to declare the pointer is the same asterisk that you use for multiplication. However, in this statement, the asterisk is being used to designate a variable as a pointer. The following are valid pointer declarations: 15.POINTERS
- 100. Go Programming 93 var ip *int /* pointer to an integer */ var fp *float32 /* pointer to a float */ The actual data type of the value of all pointers, whether integer, float, or otherwise., is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to. How toUsePointers? There are a few important operations, which we frequently perform with pointers: (a) we define pointer variables, (b) assign the address of a variable to a pointer, and (c) access the value at the address stored in the pointer variable. All these operations are carried out using the unary operator * that returns the value of the variable located at the address specified by its operand. The following example demonstrates how to perform these operations: package main import "fmt" func main() { var a int= 20 /* actual variable declaration */ var ip *int /* pointer variable declaration */ ip = &a /* store address of a in pointer variable*/ fmt.Printf("Address of a variable: %xn", &a ) /* address stored in pointer variable */ fmt.Printf("Address stored in ip variable: %xn", ip ) /* access the value using the pointer */ fmt.Printf("Value of *ip variable: %dn", *ip ) } When the above code is compiled and executed, it produces the following result: Address of var variable: 10328000
- 101. Go Programming 94 Address stored in ip variable: 10328000 Value of *ip variable: 20 NilPointersinGo Go compiler assign a nil value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned nil is called a nil pointer. The nil pointer is a constant with a value of zero defined in several standard libraries. Consider the following program: package main import "fmt" func main() { var ptr *int fmt.Printf("The value of ptr is : %xn", ptr ) } When the above code is compiled and executed, it produces the following result: The value of ptr is 0 On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the nil (zero) value, it is assumed to point to nothing. To check for a nil pointer you can use if statement as follows: if(ptr != nil) /* succeeds if p is not nil */ if(ptr == nil) /* succeeds if p is null */ GoPointersinDetail Pointers have many but easy concepts and they are very important to Go programming. The following concepts of pointers should be clear to a Go programmer:
- 102. Go Programming 95 Concept Description Go – Array of pointers You can define arrays to hold a number of pointers. Go – Pointer to pointer Go allows you to have pointer on a pointer and so on. Passing pointers to functions in Go Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function. Go–ArrayofPointers Before we understand the concept of arrays of pointers, let us consider the following example, which makes use of an array of 3 integers: package main import "fmt" const MAX int = 3 func main() { a := []int{10,100,200} var i int for i = 0; i < MAX; i++ { fmt.Printf("Value of a[%d] = %dn", i, a[i] ) } } When the above code is compiled and executed, it produces the following result: Value of a[0] = 10 Value of a[1] = 100 Value of a2] = 200
- 103. Go Programming 96 There may be a situation when we want to maintain an array, which can store pointers to an int or string or any other data type available. The following statement declares an array of pointers to an integer: var ptr [MAX]*int; This declares ptr as an array of MAX integer pointers. Thus, each element in ptr, now holds a pointer to an int value. The following example makes use of three integers, which will be stored in an array of pointers as follows: package main import "fmt" const MAX int = 3 func main() { a := []int{10,100,200} var i int var ptr [MAX]*int; for i = 0; i < MAX; i++ { ptr[i] = &a[i] /* assign the address of integer. */ } for i = 0; i < MAX; i++ { fmt.Printf("Value of a[%d] = %dn", i,*ptr[i] ) } } When the above code is compiled and executed, it produces the following result: Value of a[0] = 10 Value of a[1] = 100 Value of a[2] = 200 Go–PointertoPointer A pointer to a pointer is a form of chain of pointers. Normally, a pointer contains the address of a variable. When we define a pointer to a pointer, the first pointer contains the address of the second pointer, which points to the location that contains the actual value as shown below.
- 104. Go Programming 97 A variable that is a pointer to a pointer must be declared as such. This is done by placing an additional asterisk in front of its name. For example, the following statement declares a pointer to a pointer of type int: var ptr **int; When a target value is indirectly pointed to by a pointer to a pointer, accessing that value requires that the asterisk operator be applied twice, as is shown in the following example: package main import "fmt" func main() { var a int var ptr *int var pptr **int a = 3000 /* take the address of var */ ptr = &a /* take the address of ptr using address of operator & */ pptr = &ptr /* take the value using pptr */ fmt.Printf("Value of a = %dn", a ) fmt.Printf("Value available at *ptr = %dn", *ptr ) fmt.Printf("Value available at **pptr = %dn", **pptr) }
- 105. Go Programming 98 When the above code is compiled and executed, it produces the following result: Value of var = 3000 Value available at *ptr = 3000 Value available at **pptr = 3000 Go–PassingPointerstoFunctions Go programming language allows you to pass a pointer to a function. To do so, simply declare the function parameter as a pointer type. In the following example, we pass two pointers to a function and change the value inside the function which reflects back in the calling function: package main import "fmt" func main() { /* local variable definition */ var a int = 100 var b int= 200 fmt.Printf("Before swap, value of a : %dn", a ) fmt.Printf("Before swap, value of b : %dn", b ) /* calling a function to swap the values. * &a indicates pointer to a ie. address of variable a and * &b indicates pointer to b ie. address of variable b. */ swap(&a, &b); fmt.Printf("After swap, value of a : %dn", a ) fmt.Printf("After swap, value of b : %dn", b ) } func swap(x *int, y *int) { var temp int
- 106. Go Programming 99 temp = *x /* save the value at address x */ *x = *y /* put y into x */ *y = temp /* put temp into y */ } When the above code is compiled and executed, it produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :200 After swap, value of b :100
- 107. Go Programming 100 Go arrays allow you to define variables that can hold several data items of the same kind. Structure is another user-defined data type available in Go programming, which allows you to combine data items of different kinds. Structures are used to represent a record. Suppose you want to keep track of the books in a library. You might want to track the following attributes of each book: Title Author Subject Book ID In such a scenario, structures are highly useful. DefiningaStructure To define a structure, you must use type and struct statements. The struct statement defines a new data type, with multiple members for your program. The type statement binds a name with the type which is struct in our case. The format of the struct statement is as follows: type struct_variable_type struct { member definition; member definition; ... member definition; } Once a structure type is defined, it can be used to declare variables of that type using the following syntax. Variable_name := structure_variable_type {value1, value2…valuen} AccessingStructureMembers To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. The following example explains how to use a structure: 16.STRUCTURES
- 108. Go Programming 101 package main import "fmt" type Books struct { title string author string subject string book_id int } func main() { var Book1 Books /* Declare Book1 of type Book */ var Book2 Books /* Declare Book2 of type Book */ /* book 1 specification */ Book1.title = "Go Programming" Book1.author = "Mahesh Kumar" Book1.subject = "Go Programming Tutorial" Book1.book_id = 6495407 /* book 2 specification */ Book2.title = "Telecom Billing" Book2.author = "Zara Ali" Book2.subject = "Telecom Billing Tutorial" Book2.book_id = 6495700 /* print Book1 info */ fmt.printf( "Book 1 title : %sn", Book1.title) fmt.printf( "Book 1 author : %sn", Book1.author) fmt.printf( "Book 1 subject : %sn", Book1.subject) fmt.printf( "Book 1 book_id : %dn", Book1.book_id) /* print Book2 info */
- 109. Go Programming 102 fmt.printf( "Book 2 title : %sn", Book2.title) fmt.printf( "Book 2 author : %sn", Book2.author) fmt.printf( "Book 2 subject : %sn", Book2.subject) fmt.printf( "Book 2 book_id : %dn", Book2.book_id) } When the above code is compiled and executed, it produces the following result: Book 1 title : Go Programming Book 1 author : Mahesh Kumar Book 1 subject : Go Programming Tutorial Book 1 book_id : 6495407 Book 2 title : Telecom Billing Book 2 author : Zara Ali Book 2 subject : Telecom Billing Tutorial Book 2 book_id : 6495700 StructuresasFunctionArguments You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the same way as you did in the above example: package main import "fmt" type Books struct { title string author string subject string book_id int } func main() { var Book1 Books /* Declare Book1 of type Book */ var Book2 Books /* Declare Book2 of type Book */
- 110. Go Programming 103 /* book 1 specification */ Book1.title = "Go Programming" Book1.author = "Mahesh Kumar" Book1.subject = "Go Programming Tutorial" Book1.book_id = 6495407 /* book 2 specification */ Book2.title = "Telecom Billing" Book2.author = "Zara Ali" Book2.subject = "Telecom Billing Tutorial" Book2.book_id = 6495700 /* print Book1 info */ printBook(Book1) /* print Book2 info */ printBook(Book2) } func printBook( book Books ) { fmt.printf( "Book title : %sn", book.title); fmt.printf( "Book author : %sn", book.author); fmt.printf( "Book subject : %sn", book.subject); fmt.printf( "Book book_id : %dn", book.book_id); } When the above code is compiled and executed, it produces the following result: Book title : Go Programming Book author : Mahesh Kumar Book subject : Go Programming Tutorial Book book_id : 6495407 Book title : Telecom Billing Book author : Zara Ali Book subject : Telecom Billing Tutorial
- 111. Go Programming 104 Book book_id : 6495700 PointerstoStructures You can define pointers to structures in the same way as you define pointer to any other variable as follows: var struct_pointer *Books Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the “&” operator before the structure name as follows: struct_pointer = &Book1; To access the members of a structure using a pointer to that structure, you must use the "." operator as follows: struct_pointer.title; Let us re-write the above example using structure pointer: package main import "fmt" type Books struct { title string author string subject string book_id int } func main() { var Book1 Books /* Declare Book1 of type Book */ var Book2 Books /* Declare Book2 of type Book */ /* book 1 specification */ Book1.title = "Go Programming" Book1.author = "Mahesh Kumar"
- 112. Go Programming 105 Book1.subject = "Go Programming Tutorial" Book1.book_id = 6495407 /* book 2 specification */ Book2.title = "Telecom Billing" Book2.author = "Zara Ali" Book2.subject = "Telecom Billing Tutorial" Book2.book_id = 6495700 /* print Book1 info */ printBook(&Book1) /* print Book2 info */ printBook(&Book2) } func printBook( book *Books ) { fmt.printf( "Book title : %sn", book.title); fmt.printf( "Book author : %sn", book.author); fmt.printf( "Book subject : %sn", book.subject); fmt.printf( "Book book_id : %dn", book.book_id); } When the above code is compiled and executed, it produces the following result: Book title : Go Programming Book author : Mahesh Kumar Book subject : Go Programming Tutorial Book book_id : 6495407 Book title : Telecom Billing Book author : Zara Ali Book subject : Telecom Billing Tutorial Book book_id : 6495700
- 113. Go Programming 106 Go Slice is an abstraction over Go Array. Go Array allows you to define variables that can hold several data items of the same kind but it does not provide any inbuilt method to increase its size dynamically or get a sub-array of its own. Slices overcome this limitation. It provides many utility functions required on Array and is widely used in Go programming. Definingaslice To define a slice, you can declare it as an array without specifying its size. Alternatively, you can use make function to create a slice. var numbers []int /* a slice of unspecified size */ /* numbers == []int{0,0,0,0,0}*/ numbers = make([]int,5,5) /* a slice of length 5 and capacity 5*/ len()andcap()functions A slice is an abstraction over array. It actually uses arrays as an underlying structure. The len() function returns the elements presents in the slice where cap() function returns the capacity of the slice (i.e., how many elements it can be accommodate). The following example explains the usage of slice: package main import "fmt" func main { var numbers = make([]int,3,5) printSlice(numbers) } func printSlice(x []int){ fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x) } When the above code is compiled and executed, it produces the following result: 17.SLICES
- 114. Go Programming 107 len=3 cap=5 slice=[0 0 0] Nilslice If a slice is declared with no inputs, then by default, it is initialized as nil. Its length and capacity are zero. For example: package main import "fmt" func main { var numbers []int printSlice(numbers) if(numbers == nil){ fmt.printf("slice is nil") } } func printSlice(x []int){ fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x) } When the above code is compiled and executed, it produces the following result: len=0 cap=0 slice=[] slice is nil Subslicing Slice allows lower-bound and upper bound to be specified to get its subslice using [lower-bound:upper-bound]. For example:
- 115. Go Programming 108 package main import "fmt" func main { /* create a slice */ numbers := []int{0,1,2,3,4,5,6,7,8} printSlice(numbers) /* print the original slice */ fmt.Println("numbers ==", numbers) /* print the sub slice starting from index 1(included) to index 4(excluded)*/ fmt.Println("numbers[1:4] ==", numbers[1:4]) /* missing lower bound implies 0*/ fmt.Println("numbers[:3] ==", numbers[:3]) /* missing upper bound implies len(s)*/ fmt.Println("numbers[4:] ==", numbers[4:]) numbers1 := make([]int,0,5) printSlice(numbers1) /* print the sub slice starting from index 0(included) to index 2(excluded) */ number2 := numbers[:2] printSlice(number2) /* print the sub slice starting from index 2(included) to index 5(excluded) */ number3 := numbers[2:5] printSlice(number3) }
- 116. Go Programming 109 func printSlice(x []int){ fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x) } When the above code is compiled and executed, it produces the following result: len=9 cap=9 slice=[0 1 2 3 4 5 6 7 8] numbers == [0 1 2 3 4 5 6 7 8] numbers[1:4] == [1 2 3] numbers[:3] == [0 1 2] numbers[4:] == [4 5 6 7 8] len=0 cap=5 slice=[] len=2 cap=9 slice=[0 1] len=3 cap=7 slice=[2 3 4] append()andcopy()Functions One can increase the capacity of a slice using the append() function. Using copy() function, the contents of a source slice are copied to a destination slice. For example: package main import "fmt" func main { var numbers []int printSlice(numbers) /* append allows nil slice */ numbers = append(numbers, 0) printSlice(numbers) /* add one element to slice*/ numbers = append(numbers, 1) printSlice(numbers)
- 117. Go Programming 110 /* add more than one element at a time*/ numbers = append(numbers, 2,3,4) printSlice(numbers) /* create a slice numbers1 with double the capacity of earlier slice*/ numbers1 := make([]int, len(numbers), (cap(numbers))*2) /* copy content of numbers to numbers1 */ copy(numbers1,numbers) printSlice(numbers1) } func printSlice(x []int){ fmt.printf("len=%d cap=%d slice=%vn",len(x),cap(x),x) } When the above code is compiled and executed, it produces the following result: len=0 cap=0 slice=[] len=1 cap=2 slice=[0] len=2 cap=2 slice=[0 1] len=5 cap=8 slice=[0 1 2 3 4] len=5 cap=16 slice=[0 1 2 3 4]
- 118. Go Programming 111 The range keyword is used in for loop to iterate over items of an array, slice, channel or map. With arrays and slices, it returns the index of the item as integer. With maps, it returns the key of the next key-value pair. Range either returns one value or two. If only one value is used on the left of a range expression, it is the 1st value in the following table. Range expression 1st Value 2nd Value(Optional) Array or slice a [n]E index i int a[i] E String s string type index i int rune int map m map[K]V key k K value m[k] V channel c chan E element e E none Example The following paragraph shows how to use range: package main import "fmt" func main { /* create a slice */ numbers := []int{0,1,2,3,4,5,6,7,8} /* print the numbers */ for i:= range numbers { fmt.Println("Slice item",i,"is",numbers[i]) } /* create a map*/ coutryCapitalMap := map[string] string {"France":"Paris","Italy":"Rome","Japan":"Tokyo"} 18.RANGE
- 119. Go Programming 112 /* print map using keys*/ for country := range countryCapitalMap { fmt.Println("Capital of",country,"is",countryCapitalMap[country]) } /* print map using key-value*/ for country,capital := range countryCapitalMap { fmt.Println("Capital of",country,"is",capital) } } When the above code is compiled and executed, it produces the following result: Slice item 0 is 0 Slice item 1 is 1 Slice item 2 is 2 Slice item 3 is 3 Slice item 4 is 4 Slice item 5 is 5 Slice item 6 is 6 Slice item 7 is 7 Slice item 8 is 8 Capital of France is Paris Capital of Italy is Rome Capital of Japan is Tokyo Capital of France is Paris Capital of Italy is Rome Capital of Japan is Tokyo
- 120. Go Programming 113 Go provides another important data type named map which maps unique keys to values. A key is an object that you use to retrieve a value at a later date. Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key. DefiningaMap You must use make function to create a map. /* declare a variable, by default map will be nil*/ var map_variable map[key_data_type]value_data_type /* define the map as nil map can not be assigned any value*/ map_variable = make(map[key_data_type]value_data_type) Example The following example illustrates how to create and use a map: package main import "fmt" func main { var coutryCapitalMap map[string]string /* create a map*/ coutryCapitalMap = make(map[string]string) /* insert key-value pairs in the map*/ countryCapitalMap["France"] = "Paris" countryCapitalMap["Italy"] = "Rome" countryCapitalMap["Japan"] = "Tokyo" countryCapitalMap["India"] = "New Delhi" /* print map using keys*/ 19.MAPS
- 121. Go Programming 114 for country := range countryCapitalMap { fmt.Println("Capital of",country,"is",countryCapitalMap[country]) } /* test if entry is present in the map or not*/ captial, ok := countryCapitalMap["United States"] /* if ok is true, entry is present otherwise entry is absent*/ if(ok){ fmt.Println("Capital of United States is", capital) }else { fmt.Println("Capital of United States is not present") } } When the above code is compiled and executed, it produces the following result: Capital of India is New Delhi Capital of France is Paris Capital of Italy is Rome Capital of Japan is Tokyo Capital of United States is not present delete()Function delete() function is used to delete an entry from a map. It requires the map and the corresponding key which is to be deleted. For example: package main import "fmt" func main { /* create a map*/ coutryCapitalMap := map[string] string {"France":"Paris","Italy":"Rome","Japan":"Tokyo","India":"New Delhi"} fmt.Println("Original map")
- 122. Go Programming 115 /* print map */ for country := range countryCapitalMap { fmt.Println("Capital of",country,"is",countryCapitalMap[country]) } /* delete an entry */ delete(countryCapitalMap,"France"); fmt.Println("Entry for France is deleted") fmt.Println("Updated map") /* print map */ for country := range countryCapitalMap { fmt.Println("Capital of",country,"is",countryCapitalMap[country]) } } When the above code is compiled and executed, it produces the following result: Original Map Capital of France is Paris Capital of Italy is Rome Capital of Japan is Tokyo Capital of India is New Delhi Entry for France is deleted Updated Map Capital of India is New Delhi Capital of Italy is Rome Capital of Japan is Tokyo
- 123. Go Programming 116 Recursion is the process of repeating items in a self-similar way. The same concept applies in programming languages as well. If a program allows to call a function inside the same function, then it is called a recursive function call. Take a look at the following example: func recursion() { recursion() /* function calls itself */ } func main() { recursion() } The Go programming language supports recursion. That is, it allows a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go on to become an infinite loop. ExamplesofRecursioninGo Recursive functions are very useful to solve many mathematical problems such as calculating factorial of a number, generating a Fibonacci series, etc. Example1:CalculatingFactorialUsingRecursioninGo The following example calculates the factorial of a given number using a recursive function: package main import "fmt" func factorial(i int) { if(i <= 1) { return 1 } return i * factorial(i - 1) } 20.RECURSION
- 124. Go Programming 117 func main { var i int = 15 fmt.Printf("Factorial of %d is %dn", i, factorial(i)) } When the above code is compiled and executed, it produces the following result: Factorial of 15 is 2004310016 Example2:FibonacciSeriesUsingRecursioninGo The following example shows how to generate a Fibonacci series of a given number using a recursive function: package main import "fmt" func fibonaci(i int) { if(i == 0) { return 0 } if(i == 1) { return 1 } return fibonaci(i-1) + fibonaci(i-2) } func main() { var i int for i = 0; i < 10; i++ { fmt.Printf("%dt%n", fibonaci(i)) } } When the above code is compiled and executed, it produces the following result:
- 125. Go Programming 118 0 1 1 2 3 5 8 13 21 34
- 126. Go Programming 119 Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a long value into a simple integer then you can type cast long to int. You can convert values from one type to another using the cast operator. Its syntax is as follows: type_name(expression) Example Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating number operation. package main import "fmt" func main() { var sum int = 17 var count int = 5 var mean float32 maen = float32(sum)/float32(count) fmt.Printf("Value of mean : %fn",mean) } When the above code is compiled and executed, it produces the following result: Value of mean : 3.400000 21.TYPE CASTING
- 127. Go Programming 120 Go programming provides another data type called interfaces which represents a set of method signatures. The struct data type implements these interfaces to have method definitions for the method signature of the interfaces. Syntax /* define an interface */ type interface_name interface { method_name1 [return_type] method_name2 [return_type] method_name3 [return_type] ... method_namen [return_type] } /* define a struct */ type struct_name struct { /* variables */ } /* implement interface methods*/ func (struct_name_variable struct_name) method_name1() [return_type] { /* method implementation */ } ... func (struct_name_variable struct_name) method_namen() [return_type] { /* method implementation */ } 22.INTERFACES
- 128. Go Programming 121 Example package main import ( "fmt" "math" ) /* define an interface */ type Shape interface { area() float64 } /* define a circle */ type Circle struct { x,y,radius float64 } /* define a rectangle */ type Rectangle struct { width, height float64 } /* define a method for circle (implementation of Shape.area())*/ func(circle Circle) area() float64 { return math.Pi * circle.radius * circle.radius } /* define a method for rectangle (implementation of Shape.area())*/ func(rect Rectangle) area() float64 { return rect.width * rect.height } /* define a method for shape */
- 129. Go Programming 122 func getArea(shape Shape) float64 { return shape.area() } func main() { circle := Circle{x:0,y:0,radius:5} rectangle := Rectangle {width:10, height:5} fmt.Printf("Circle area: %fn",getArea(circle)) fmt.Printf("Rectangle area: %fn",getArea(rectangle)) } When the above code is compiled and executed, it produces the following result: Circle area: 78.539816 Rectangle area: 50.000000
- 130. Go Programming 123 Go programming provides a pretty simple error handling framework with inbuilt error interface type of the following declaration: type error interface { Error() string } Functions normally return error as last return value. Use errors.New to construct a basic error message as following: func Sqrt(value float64)(float64, error) { if(value < 0){ return 0, errors.New("Math: negative number passed to Sqrt") } return math.Sqrt(value) } Use return value and error message. result, err:= Sqrt(-1) if err != nil { fmt.Println(err) } Example package main import "errors" import "fmt" import "math" func Sqrt(value float64)(float64, error) { if(value < 0){ 23.ERROR HANDLING
- 131. Go Programming 124 return 0, errors.New("Math: negative number passed to Sqrt") } return math.Sqrt(value) } func main() { result, err:= Sqrt(-1) if err != nil { fmt.Println(err) }else { fmt.Println(result) } result, err = Sqrt(9) if err != nil { fmt.Println(err) }else { fmt.Println(result) } } When the above code is compiled and executed, it produces the following result: Math: negative number passed to Sqrt 3