LOOPS

A loop is a structure that allows repeated execution of a block of statements. Java has three basic loops:

while loop

You use the while loop to execute a body of statements continuously while some conditions continue to be true.

Syntax                              while (condition)
                                       {

                                            loop body;

                                        }                     

Within a while loop structure, a boolean expression  is evaluated. If it is true, then a block statements, called the loop body, executes, then the boolean expression is evaluated again. As long as the expression is true, the statements in the loop body continue to execute. When the boolean expression is false, the loop ends. If the boolean expression is never false, an infinite loop will occur.

Example Output Comments
count=1;
while(count < 3)
{ System.out.println("Hello");
count=count+1;
}		 Hello
Hello		 count is called a loop control variable. The loop control variable can be increment or decrement.

                                                       

do ... while LOOP

The do ..... while loop is a loop that checks the bottom of the loop after one repetition has occurred. If you want to perform a task at least one time, the do ... while loop is a solution.

Syntax                 do
                           {   loop body;
                           }while(boolean expression);

Within the do ... while loop, the loop body is executed first, then the boolean expression is evaluated. If it is true, then a block statements, called the loop body, executes, then the boolean expression is evaluated again. As long as the expression is true, the statements in the loop body continue to execute. When the boolean expression is false, the loop ends. If the boolean expression is never false, an infinite loop will occur.

 

Example Output Comments
count=1;
do
{ System.out.println("Hello");
count=count+1;
}while(count< 4);		 Hello
Hello
Hello		 count is called a loop control variable. The loop control variable can be increment or decrement.

for LOOP

You can use a while loop when you need to perform a task   some predetemined number of times. A loop that executes a specific number of times is a definite loop or counted loop. for loop is a definite loop.

Syntax       for (expression1;expression2;expression3)
                 {

                          loop body;

                 }

The following steps show how the for loop is executed.

  1. The expression1 executes. Program control passes to step 2.
  2. If the expression2 (a boolean expression) evaluates to true, then the loop body is executed. After the loop body executes, the program control passes to step 3. If the boolean expression evaluates to false, the program control exits the loop.
  3. The expression3 is evaluated, then the program control passes to step 2.
Example Output
for (int i=0 ;  i<3 ; i++)
{  System.out.println("hello");
}		 hello
hello
hello

You can write a for loop that do not include any or all of the three expressions, but you always must include the semicolons.

Example Output
int i=0;
for ( ;  i<3 ; i++)
{  System.out.println("hello");
}		 hello
hello
hello
Example Output
for ( int i=0 ;   ;  i++)
{  System.out.println("hello");
}		 hello
hello
hello
hello
.
.

 
Example Output
for (int i=0 ;  i<3 ; )
{  System.out.println("hello");
    i++;
}		 hello
hello
hello
Example Output
for ( ;  ; )
{  System.out.println("hello");
}		 hello
hello
hello
.
.

break STATEMENT

You use a break statement in a Java program to exit from a while loop, a do while loop, or a for loop. The break statement causes an exit from only the innermost loop; it will not exit multiple loops. Before the break statement, it is often a condition statement.

Example Output
import javax.swing.*;
public class Test
{ public static void main(String[ ] args)
   {   int num, test|;
        String s;
        do
       {  s = JOptionPane.showInputDialog("Enter a number");
          num = Integer.parseInt(s);
   test=num % 2;
   if ( test = = 0)
       break;
   System.out.println(num +" is odd.");
      }while( 1 );    
System.out.println(num + " is even");
}
}		 number
1
1 is odd.
number
5
5 is odd.
number
6
6 is even

 

Note:  while ( 1 ) means that the condition of loop is always true. Remember in Java any integer except 0 is a true value.

Counter-controlled loop

Write a while loop that displays Hello 100 times.

int  i = 0;
while ( i < 100)
{ System.out.println(i);
   i = i + 1;
}
 

Sentinel-value controlled loop

Accumulate all the integer numbers entered from the user until the input is 0. The number 0 is the sentinel-value.

int num, sum = 0;
String s;
s = JOptionPane.showInputDialog("Enter a number");
num = Integer.parseInt(s);
while (num != 0)
{ sum = sum + num; 
   s = JOptionPane.showInputDialog("Enter a number");
          num = Integer.parseInt(s);
 }
 System.out.println(sum); 

Flag-controlled loop

int num , sum =0;
String s;
boolean flag =  true;
while( flag)
{
    s=JOptionPane.showInputDialog(“Enter a number”);
    num= Integer.parseInt(s);
   sum= sum + num ;
   if( sum>30)
         flag=false ;
} // Close the loop
System.out.println(sum);

Example
Display Hello 10 times

Use a while loop

int  i= 1;
while (i<10)
{System.out.println(“hello”);
i=i+1;
}

Use a for loop 

for (int i= 1; i<=10;i=  i+1 )
      System.out.println( “Hello”);

Example write a for loop to display even numbers starting from 4 to 60

for( int i = 4 ; i < = 60 ; i = i +2 )
            System.out.println(i);