Comp102 lec 6

Java in two semesters by Quentin Charatan & Aaron Kans

 Computers can repeat the same tasks over and
over, again and again
 Form of program control that allows us to instruct
the computer to carry out a task by repeating a
section of code
 Iteration
 Loop
 Types of Loops
 for loop
 while loop
 do...while loop

System.out.println(“*****”);
***** System.out.println(“*****”);
***** Loop 1…5
End-Loop
for(int i=0; i<5; i++)
{
}

10
9 for(int i=10; i>=1; i--)
8 {
7 System.out.println(i);
6 }
5
4
3
2
1

2
4
for(int i=1; i<=20;i++)
6
{
8
if(i%2 ==0)
10
{
12
System.out.println(i);
14
}
16
}
18
20

for(int i=0; i<5; i++)
***** {
***** for(int j = 0; j<5; j++
***** {
***** System.out.print(“*”);
System.out.println(“*”);
***** }
} System.out.println();
}

 Often used to implement the fixed repetitions

 Consider the following scenarios
 A racing game that repeatedly moves a car around
until the car crashes
 A ticket issuing program that repeatedly offers tickets
for sale until he user chooses to quit the program
 A password checking program that does not let a user
enter into an application until he or she enters the
right password
 Thenumber of repetitions is not fiexed but
depends on some condition
 Non fixed Iterations

while (/*test goes here*/)
{
//instruction(s) to be repeated
}
 simpler to construct
 If not used as counter loop, then it is not required
to keep a counter
 Normally used to validate the input

import java.util.Scanner;
public class Assignment1
{
public static void main(String[] args)
{
int marks;
Scanner input = new Scanner(System.in);
marks = input.nextInt();
while(marks<0 || marks>100)
{
System.out.println("Invalid Marks - ReEnter?");
marks = input.nextInt();
}
if(marks>=40)
{
System.out.println("Congratulations... You passed");
}
else
{
System.out.println("Sorry you failed the course");
}
}
}

int i=0;
while(i<5)
*****
{
*****
int j = 0;
*****
while(j<5)
*****
{
*****
System.out.print(“*”);
j++;
}
System.out.println();
i++;
}

 Tests the condition at beginning
 If the condition is false at start, makes the loop to
not execute ever
 Loop executes ZERO or MORE times

 Non – Fixed Loop
 Condition is tested at the end of the loop
 Makes the loop to iterate at least once
 Makes the loop to iterate ONE or MORE times
 while loop terminates once the program has done
its job.
 Do while is suitable if we wish to re-run the same
program again based on user response
 Syntax:

do
{
//instruction(s) to be repeated goes here
}while (/* test goes here*/);

char response;
do
{
Scanner input = new Scanner(System.in);
//program instructions go here
System.out.println(“Want to re-run? (y/n)”);
response = input.next().charAt(0);
}while(response ==‘y’);

do
{
System.out.println("1 - for group A");
System.out.println("2 - for group B");
System.out.println("3 - Quit");
response = sc.nextInt();
switch(response)
{
case 1:
System.out.println("10:00 AM");
break;
case 2:
System.out.println("11:00 AM");
break;
case 3:
System.out.println("Good Bye");
break;
default:
System.out.println("Invalid Input");
break;
}
}while(response!= 3);

 For Loop
 Number of repetitions required can be determined
prior to entering the loop
 While Loop
 Number of repetitions cannot be determined prior to
entering the loop and
 Zero repetitions is allowed
 Do… while loop
 Number of repetitions cannot be determined before
the loop
 You require at least one repetition of loop

Comp102 lec 6

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Comp102 lec 6