LECTURE 3 LOOPS, ARRAYS.pdf

6.092: Intro to Java

3: Loops, Arrays

Assignment 2

Foo Corporation needs a program to calculate
how much to pay their employees.
1. Pay = hours worked x base pay
2. Hours over 40 get paid 1.5 the base pay
3. The base pay must be no less than $8.00
4. The number of hours must be no more than 60

Frequent Issues (I)

The signature of the main method cannot be
modified.
public static void main(String[] arguments) {
...
}

Frequent Issues (II)

Return values: if you declare that the method is not void,
then it has to return something!
public static int pay(double basePay, int hours) {
if (basePay < 8.0) return -1;
else if (hours > 60) return -1;
else {
int salary = 0;

...

return salary

}
}

Frequent Issues (III)

Don't create duplicate variables with the same
name
public static int pay(double basePay, int hours) {

int salary = 0; // OK
…
int salary = 0; // salary already defined!!
…
double salary = 0; //salary already defined!!
…
}

class WeeklyPay {
public static void pay(double basePay, int hours) {
if (basePay < 8.0) {
System.out.println("You must be paid at least $8.00/hour");
} else if (hours > 60) {
System.out.println("You can't work more than 60 hours a week");
} else {

int overtimeHours = 0;

if (hours > 40) {

overtimeHours = hours - 40;
hours = 40;
}
double pay = basePay * hours;
pay += overtimeHours * basePay * 1.5;
System.out.println("Pay this employee $" + pay);
}
}
public static void main(String[] arguments) {
pay(7.5, 35);
pay(8.2, 47);
pay(10.0, 73);
}
}

What we have learned so far

● Variables & types
● Operators
● Type conversions & casting

● Methods & parameters
● If statement

Today’s Topics
● Good programming style

● Loops
● Arrays

Good programming style

The goal of good style is to make your

code more readable.

By you and by others.

Rule #1: use good (meaningful) names

String a1;

int a2;

double b; // BAD!!

String firstName; // GOOD

String lastName; // GOOD

int temperature; // GOOD

Rule #2: Use indentation
public static void main (String[] arguments) {
int x = 5;
x = x * x;
if (x > 20) {
System.out.println(x + “ is greater than 20.”);
}
double y = 3.4;
}
Have a demo with no indentation
Ctrl-shift-F to auto-format the file

Rule #3: Use whitespaces
Put whitespaces in complex expressions:
// BAD!!
double cel=fahr*42.0/(13.0-7.0);
// GOOD
double cel = fahr * 42.0 / (13.0 - 7.0);

Rule #3: Use whitespaces
Put blank lines to improve readability:
public static void main (String[] arguments) {

int x = 5;

x = x * x;

if (x > 20) {
System.out.println(x + “ is > 20.”);
}
double y = 3.4;
}

Rule #4: Do not duplicate tests
...
...
} else if (basePay >= 8.0 && hours <= 60) {
...
}

...
...
} else if (basePay >= 8.0 && hours <= 60){
...
}
BAD


...

...

} else {
...
}

Good programming style (summary)

Use good names for variables and methods
Use indentation
Add whitespaces
Don't duplicate tests

Loops
static void main (String[] arguments) {
System.out.println(“Rule #1”);
}
What if you want to do it for 200 Rules?

Loops
Loop operators allow to loop through a block
of code.
There are several loop operators in Java.

The while operator
while (condition) {

statements
}

The while operator
int i = 0;
while (i < 3) {
System.out.println(“Rule #“ + i);
i = i+1;
}
Count carefully
Make sure that your loop has a chance to finish.

The for operator

for (initialization;condition;update){
statements
}

The for operator
for (int i = 0; i < 3; i=i+1) {
System.out.println(“Rule #“ + i);
}
Note: i = i+1 may be replaced by i++

S
}
Branching Statements
break terminates a for or while loop
break;
ystem.out.println(“Rule #” + i);
for (int i=0; i<100; i++) {
if(i == 50)

r
Branching Statements

continue skips the current iteration of a loop
and proceeds directly to the next iteration
fo (int i=0; i<100; i++) {
if(i == 50)
continue;
System.out.println(“Rule #” + i);
}

Embedded loops
for (int i = 0; i < 3; i++) {
for (int j = 2; j < 4; j++) {
System.out.println (i + “ “ + j);
}
}
Scope of the variable defined in the initialization:
respective for block

Arrays

An array is an indexed list of values.
You can make an array of any type
int, double, String, etc..
All elements of an array must have the same type.

Arrays

Example: double [ ]

5.0 2.44 9.01 1.0 -9.9
..
0 1 2 3 .. n-1

Arrays
The index starts at zero and ends at length-1.
Example:
int[] values = new int[5];
values[0] = 12; // CORRECT
values[4] = 12; // CORRECT
values[5] = 12; // WRONG!! compiles but
// throws an Exception
// at run-time
Have a demo with runtime exception

Arrays
An array is defined using TYPE[].
Arrays are just another type.
int[] values; // array of int
int[][] values; // int[] is a type

Arrays

To create an array of a given size, use the operator new :
or you may use a variable to specify the size:
int size = 12;
int[] values = new int[size];

Array Initialization

Curly braces can be used to initialize an array.
It can ONLY be used when you declare the
variable.
int[] values = { 12, 24, -23, 47 };

Quiz time!

Is there an error in this code?
int[] values = {1, 2.5, 3, 3.5, 4};

Accessing Arrays

To access the elements of an array, use the [] operator:
values[index]
Example:
int[] values = { 12, 24, -23, 47 };
values[3] = 18; // {12,24,-23,18}
int x = values[1] + 3; // {12,24,-23,18}

The length variable

Each array has a length variable built-in that
containsthe length of the array.

int size = values.length; // 12

int[] values2 = {1,2,3,4,5}

int size2 = values2.length; // 5

String arrays

A side note

public static void main (String[] arguments){
System.out.println(arguments.length);
System.out.println(arguments[0]);
System.out.println(arguments[1]);
}

Looping through an array
Example 1:
for (int i=0; i<values.length; i++) {
values[i] = i;
int y = values[i] * values[i];
System.out.println(y);
}

Looping through an array
Example 2:

int i = 0;

while (i < values.length) {

values[i] = i;
int y = values[i] * values[i];
System.out.println(y);
i++;

}

Summary for today

1. Programming Style

2. Loops
3. Arrays

Assignment 3
A group of friends participate in the Boston

Marathon.

Find the best performer.

Find the second-best performer.

MIT OpenCourseWare
http://ocw.mit.edu
6.092 Introduction to Programming in Java
January (IAP) 2010
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.

LECTURE 3 LOOPS, ARRAYS.pdf

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