Unit 4 functions and pointers

FUNCTION
 A Function is a sub-program, which contains one or
more statements and it performs some task when its
called.
 A computer program cannot handle all the tasks by it
self. Instead its requests other program like entities –
called functions in C to get its tasks done.
 A function is a self contained block of statements that
perform a coherent task of same kind
KIRTHIKA KM /AP/CSE

Why we use functions?
 Writing functions avoids rewriting the same code over and
over.
 Suppose that there is a section of code in a program that
calculates area of a circle. If later in the program we want to
calculate the area of a different circle, we wont like to write
the same instructions again.
 Instead, we would prefer to jump to a “section of code” that
calculates area and then jump back to the place from where
we left off.
 This section of code is nothing but a function.
KIRTHIKA KM /AP/CSE

 Using functions it becomes easier to write programs and
keep track of what they are doing.
 If the operation of a program can be divided in to separate
activities, and each activity placed in a different function,
then each could be written and checked more or less
independently.
 Separating the code in to modular functions also makes the
pro-gram easier to design and understand.
Why we use functions?
KIRTHIKA KM /AP/CSE

TYPES
 There are two different types of functions:
 Pre-defined functions
 User-defined functions
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Pre-Defined Functions
 The pre-defined functions or library functions are
built-in functions.
 The user can use the functions, but cannot modify
those functions.
 Example: sqrt()
KIRTHIKA KM /AP/CSE

User-Defined Functions
 The functions defined by the user for their
requirements are called user-defined functions.
 Whenever it is needed, the user can modify this
function.
 Example: sum(a,b)
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Advantage of User-Defined Functions
 The length of the source program can be reduced.
 It is easy to locate errors.
 It avoids coding of repeated instructions.
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Elements of User-Defined
Function
 Function declaration
 Function definition
 Function call
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Function
 Syntax
datatype function_name (parameters list)
{
local variable declaration;
…………………………
body of the function;
…………………………
return(expression);
}
KIRTHIKA KM /AP/CSE

How Function Works?
 Once a function is called the control passes to the
called function.
 The working of calling function is temporarily
stopped.
 When the execution of called function is completed
then the control returns back to the calling function
and executes the next statement.
KIRTHIKA KM /AP/CSE

Parameters
 Actual Parameter
These are the parameters which are transferred from
the calling function to the called function.
 Formal Parameter
These are the parameters which are used in the called
function.
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return Statement
 The return statement may or may not send some
values to the calling function.
 Syntax:
return; (or)
return (expression);
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Function Prototypes
There are four types:
 Function with no arguments and no return values.
 Function with arguments and no return values.
 Function with arguments and return values.
 Function with no arguments and with return values.
KIRTHIKA KM /AP/CSE

Function with no arguments
and no return values
 Here no data transfer takes place between the calling
function and the called function.
 These functions act independently, i.e. they get input
and display output in the same block.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main() //calling function
{
void add(void);
add();
}
void add() //called function
{
int a,b,c;
printf("nEnter two number:");
scanf("%d%d",&a,&b);
c=a+b;
printf("nSum is:%d",c);
}
KIRTHIKA KM /AP/CSE

Output
Enter two number:3
4
Sum is:7
KIRTHIKA KM /AP/CSE

Function with arguments
and no return values
 Here data transfer take place between the calling
function and the called function.
 It is a one way data communication, i.e. the called
program receives data from calling program but it does
not return any value to the calling program.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int a,b;
void add(int,int);
add(a,b);
}
void add(int x,int y) //function with arguments
{
int z;
z=x+y;
printf("nSum is:%d",z);
}
KIRTHIKA KM /AP/CSE

Output
Enter two number:2
4
Sum is:6
KIRTHIKA KM /AP/CSE

Function with arguments
and return values
 Here data transfer takes place between the calling
function and the called function as well as between
called function and calling function .
 It is a two way data communication, i.e. the called
program receives data from calling program and it
returns some value to the calling program.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int a,b,c;
int add(int,int);
c=add(a,b);
printf("nSum is:%d",c);
}
int add(int x,int y)
{
int z;
z=x+y;
return(z);
}
KIRTHIKA KM /AP/CSE

Output
Enter two number:6
7
Sum is:13
KIRTHIKA KM /AP/CSE

Function with no arguments
and with return values
 Here data transfer takes place between the called
function and the calling function.
 It is a one way data communication, i.e. the called
program does not any receive data from the calling
program but it returns some value to the calling
program.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int add(),d;
d=add();
printf("nSum is:%d",d);
}
int add() //function with no argument
{
int a,b,c;
c=a+b;
return(c);
}
KIRTHIKA KM /AP/CSE

Output
Enter two number:5
8
Sum is:13
KIRTHIKA KM /AP/CSE

Parameter Passing Methods
 There are two different ways of passing parameters to a
method, they are:
 Call by value
 Call by reference
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Call by value
 Actual arguments are passed to the formal arguments.
 Any changes made to the formal argument does not
affect the actual argument.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int x,y,change(int,int);
printf("nEnter value of x:");
scanf("%d",&x);
printf("nEnter value of y:");
scanf("%d",&y);
change(x,y);
printf("nnValues in the
Main()-->x=%d,y=%d",x,y);
}
int change(int a,int b)
{
int c;
c=a;
a=b;
b=c;
printf("nValues in the
Fuction --
>x=%d,y=%d",a,b);
}
KIRTHIKA KM /AP/CSE

Output
Enter value of x:5
Enter value of y:6
Values in the Function -->x=6,y=5
Values in the Main()-->x=5,y=6
KIRTHIKA KM /AP/CSE

Call by reference
 Instead of passing values, the address of the argument
will be passed.
 Any changes made to the formal argument will affect
the actual argument.
KIRTHIKA KM /AP/CSE

Example
#include <stdio.h>
#include<conio.h>
void main()
{
int x,y,change(int*,int*);
printf("nEnter value of
x:");
scanf("%d",&x);
printf("nEnter value of
y:");
scanf("%d",&y);
change(&x,&y);
printf("nnValues in
the Main()--
>x=%d,y=%d",x,y);
}
int change(int *a,int *b)
{
int c;
c=*a;
*a=*b;
*b=c;
printf("nValues in the
Function --
>x=%d,y=%d",*a,*b);
}
KIRTHIKA KM /AP/CSE

Output
Enter value of x:5
Enter value of y:6
Values in the Function -->x=6,y=5
Values in the Main()-->x=6,y=5
KIRTHIKA KM /AP/CSE

Recursion
 It is a process of calling the same function itself again and
again until some condition is satisfied.
Syntax:
func1()
{
………..
func1();
…………
}
KIRTHIKA KM /AP/CSE

Example
#include<stdio.h>
#include<conio.h>
void main()
{
int a;
int rec(int);
printf("nEnter the number:");
scanf("%d",&a);
printf("The factorial of %d! is
%d",a,rec(a));
}
int rec(int x)
{
int f;
if(x==1)
return(1);
else
f=x*rec(x-1);
return(f);
}
Output:
Enter the number:5
The factorial of 5! is 120
KIRTHIKA KM /AP/CSE

Example: Working of 3!
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Library Function
 Library functions are the pre-defined functions.
 The library function provides functions like
mathematical, string manipulation etc,.
 In order to use a library function, it is necessary to call
the appropriate header file at the beginning of the
program.
 The header file informs the program of the name,
type, and number and type of arguments, of all of the
functions contained in the library in question.
 A header file is called via the preprocessor statement.
KIRTHIKA KM /AP/CSE

Some Examples of Library
Functions
 sqrt(x):
It is used to find the square root of x
Example: sqrt(36) is 6
 abs(x):
It is used to find the absolute value of x
Example: abs(-36) is 36
 pow(x,y):
It is used to find the value of xy
Example: pow(5,2) is 25
KIRTHIKA KM /AP/CSE

 ceil(x):
It is used to find the smallest integer greater than or equal to x.
Example: ceil(7.7) is 8
 rand():
It is used to generate a random number.
 sin(x):
It is used to find the sine value of x
Example: sin(30) is 0.5
 cos (x):
It is used to find the cosine value of x
Example: cos(30) is 0.86
KIRTHIKA KM /AP/CSE

 tan(x):
It is used to find the tan value of x
Example: tan(30) is 0.577
 toascii(x):
It is used to find the ASCII value of x
Example: toascii(a) is 97
 toupper(x):
It is used to convert lowercase character to uppercase.
Example: toupper(‘a’) is A toupper(97) is A
 tolower(x):
It is used to convert uppercase character to lowercase.
Example: tolower(‘A’) is a
KIRTHIKA KM /AP/CSE

Example:
#include<stdio.h>
#include<conio.h>
#include<math.h>
#include<ctype.h>
void main()
{
int x,y=2;
printf("nEnter the number:");
scanf("%d",&x);
printf("nThe square root of %d is %f",x,sqrt(x));
printf("nThe value of %d power%dis%f ",x,y,pow(6,2));
printf("nThe ceiling of 6.7 is %f",ceil(6.7));
printf("nThe floor of 6.7 is %f",floor(6.7));
KIRTHIKA KM /AP/CSE

printf("nThe absolute value of -6 is %d",abs(-6));
printf("nThe value of sin 45 is %f",sin(45));
printf("nThe uppercase of 'a' is %c",toupper('a'));
printf("nThe uppercase of 97 is %c",toupper(97));
getch();
}
KIRTHIKA KM /AP/CSE

Output:
Enter the number:6
The square root of 6 is 2.449490
The value of 6 power 2 is 36.000000
The ceiling of 6.7 is 7.000000
The floor of 6.7 is 6.000000
The absolute value of -6 is 6
The value of sin 45 is 0.850904
The uppercase of 'a' is A
The uppercase of 97 is A
KIRTHIKA KM /AP/CSE

Applications:
 Math functions
 Computation of Sine Series
 Random Number Generation
 Tower of Hanoi
 Factorial using Recursive functions.
KIRTHIKA KM /AP/CSE

Math functions
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Pointers
 Pointer is a variable that contains 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.
KIRTHIKA KM /AP/CSE

Example:
x=5
x Variable
1002 Address
5 Value
KIRTHIKA KM /AP/CSE

Example
#include<stdio.h>
#include<conio.h>
void main()
{
int x=5;
printf("n The Address of x = %u",&x);
printf("n The Value of x = %d",x);
}
Output
The Address of x = 8714
The Value of x = 5
KIRTHIKA KM /AP/CSE

Pointer Declaration
 Syntax:
data-type *pointer-name;
data-type - Type of the data to
which the pointer points.
pointer-name - Name of the pointer
 Example: int *a;
KIRTHIKA KM /AP/CSE

Accessing Variable through Pointer
 If a pointer is declared and assigned to a variable, then
the variable can be accessed through the pointer.
 Example:
int *a;
x=5;
a=&x;
KIRTHIKA KM /AP/CSE

Example
#include<stdio.h>
#include<conio.h>
void main()
{
int y=10;
int *a;
a=&y;
printf("n The Value of y = %d",y);
printf("n The Address of y = %u",&y);
printf("n The Value of a = %d",a);
printf("n The Address of a = %u",&a);
}
KIRTHIKA KM /AP/CSE

Illustration of the example:
5001 10
8000
a y
5001
Variable
Value
Address
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Output
The Value of y = 10
The Address of y = 5001
The Value of a = 5001
The Address of a = 8000
KIRTHIKA KM /AP/CSE

Null Pointer
 A pointer is said to be null pointer if zero is assigned to
the pointer.
 For Example:
int *a,*b;
a=b=0;
KIRTHIKA KM /AP/CSE

Pointer to Pointer
 Here one pointer stores the address of another pointer
variable.
 Example:
int x=10,*a,**b;
a=&x;
b=&a;
KIRTHIKA KM /AP/CSE

 Illustration:
5001 10
8000
a x
5001
Variable
Value
Address
8000
9000
b
KIRTHIKA KM /AP/CSE

Example
#include<stdio.h>
#include<conio.h>
void main()
{
int a=10;
int *b,**c;
b=&a;
c=&b;
printf("n The Value of a = %d",a);
printf("n The Address of a = %u",&a);
printf("n The Value of b = %d",b);
printf("n The Address of b = %u",&b);
printf("n The Value of c = %d",c);
printf("n The Address of c = %u",&c);
}
KIRTHIKA KM /AP/CSE

Output
The Value of a = 10
The Address of a = 5001
The Value of b = 5001
The Address of b = 8000
The Value of c = 8000
The Address of c = 9000
KIRTHIKA KM /AP/CSE

Pointers and Arrays
 The elements of the array can also be accessed through
a pointer.
 Example
int a[3]={2,3,7};
int *b;
b=a;
KIRTHIKA KM /AP/CSE

Example 1
#include<stdio.h>
#include<conio.h>
void main()
{
int a[3]={2,3,7};
int *b;
b=a;
printf("n The Value of a[0] = %d",a[0]);
printf("n The Address of a[0] = %u",&a[0]);
printf("n The Value of b = %d",b);
}
KIRTHIKA KM /AP/CSE

 Illustration of the example:
8744 2
9000
b a[0]
8744
Variable
Value
Address
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Output
The Value of a[0] = 2
The Address of a[0] = 8744
The Value of b = 8744
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Example 2
#include<stdio.h>
#include<conio.h>
void main()
{
int a[5]={2,3,7,9,10};
int i;
for(i=0;i<5;i++)
{
printf("n The Value of a[%d] = %d",i,a[i]);
printf("n The Address of a[%d] = %u",i,&a[i]);
}
}
KIRTHIKA KM /AP/CSE

Illustration of the example:
2 3 7 9 10
a[0] a[1] a[2] a[3] a[4]
8724 8726 8728 8730 8732
Array
Value
Address
KIRTHIKA KM /AP/CSE

Output
KIRTHIKA KM /AP/CSE

Example 3
#include<stdio.h>
#include<conio.h>
void main()
{
int a[5]={1,2,3,4,5};
int i,sum=0;
int *b;
b=a;
for(i=0;i<5;i++)
{
sum=sum + *b;
b++; //b=b+1
}
printf("n The Sum is %d",sum);
}
KIRTHIKA KM /AP/CSE

Output
The Sum is 15
KIRTHIKA KM /AP/CSE

Pointers and Structures
 Syntax:
struct structure_name
{
structure element1;
structure element2;
…………………….
}variable,*ptr;
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 A sample code illustrating pointers and structure
struct stud
{
int sno;
char name[10];
int mark;
};
struct stud *s;
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Example
#include<stdio.h>
#include<conio.h>
struct stud
{
int regno;
char name[10];
int m1;
int m2;
int m3;
};
struct stud s;
struct stud *t;
KIRTHIKA KM /AP/CSE

void main()
{
float tot,avg;
t=&s;
printf("nEnter the student regno,name,m1,m2,m3:");
scanf("%d%s%d%d%d",&s.regno,&s.name,&s.m1,&s.m2,&s.m3);
tot=s.m1+s.m2+s.m3;
avg=tot/3;
printf("nThe student Details are:");
printf("n%dt%st%ft%f",s.regno,s.name,tot,avg);
printf("n%dt%st%ft%f",t->regno,t->name,tot,avg);
}
KIRTHIKA KM /AP/CSE

Output
Enter the student regno,name,m1,m2,m3:1
aaa
76
89
76
The student Details are:
1 aaa 241.000000 80.333336
1 aaa 241.000000 80.333336
KIRTHIKA KM /AP/CSE

Unit 4 functions and pointers

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Unit 4 functions and pointers