Java Looping | While Loop

Looping

“The process of repeatedly executing a block of statements is known as looping.”

If a loop continues forever, it is called as infinite loop. Just like the sun arises on the earth every day.

In looping, statements block of statement is executed until some conditions for the termination of the loop occurred.

How looping works?

1. Setting and initialization of a counter
2. Execution of the statements in the loop
3. Test for a specified condition for execution of the loop
4. Incrementing the counter

Different types of loops?

1. while loop
2. do while loop
3. for loop

while loop

While loop is the simplest looping structure. While loop is entry controlled loop statement. That means the condition is given at the beginning of loop. If the given condition does not satisfy , the loop statements never get executed.

Syntax

while (test-condition)
{
	// body of while loop
	//increment/decrement counter
}

Flowchart

Working of while loop

• First the test condition is evaluated before entering into the while loop. (counter should be initialized outside the loop).
• If the test condition returns true result, then body of the loop is executed and again condition is checked.
• Counter increments every time untill the test condition is having true result.
• Different ways of incrementing the loop.
• count++ or ++count
• count = count + 1
• count += 1
• As soon the test condition becomes false then loop is terminated.

Example - Printing 1 to 10 numbers using while loop

class WhileLoopEx
{
   public static void  main(String args[])
   {
      int start = 1;
      int end = 10;

      while ( start <= end)
      {
        System.out.println(start);
	  start ++;
      }
   }
}

Output

Program Explanation

• First of all start variable is initialized to 1 and end to 10 (for the first time in a life of while loop).
• Before entering into the loop, while first checks the condition 1 <= 10 which is true, then control enters into the loop.
• Print start variable which is holding 1, and then increments start variable by 1, now start stores the new value 2.
• Now again condition is checked as 2 <= 10 which is true. then follow step 2,3 until 10 <=10 becomes false.
• Exit from loop.

Iterations on Example

Before entering into the loop, the value of start = 1 and value of end = 10

Different ways to write while loop

1. while loop without braces

If we have only single statement in the body of loop,then we can write it without using curly braces.

Example

class WhileLoopEx
{
   public static void  main(String args[])
   {
      int i=1,add=0;

      while(i<=5)
      add += i++;
        
      System.out.println("add = "+add);
   }
}                             

Output

2. Increment|Decrement and condition at same line

Below program shows the numbers between 1 to 5

class WhileLoopEx
{
   public static void  main(String args[])
   {
      int a=0;
      while(a++ < 5)
      {
         System.out.println("a : "+a);
      }
   }
}

Output

3. We can also check float,double etc.

Below program checks the float condition and adds float value by 1.0

class WhileLoopEx
{
   public static void  main(String args[])
   {
      float a = 1.0f;
      double add = 0;
      while(a<=5.5)
      {
         add = add +a ;
	   a=a+0.5f;
      }
      System.out.println("addition : "+add);
   }
}

Output

4. Running Indefinite loop

Below program doesn't give any output at all. due to ; after while. Compiler will keep rotating eternally.

class WhileLoopEx
{
   public static void  main(String args[])
   {
      int a=0;
      while(a < 10);
      {
        a++;
	  System.out.println("a : "+a);
      }
      System.out.println("final a : "+a);
   }
}

Output

No output will be displayed.

5. Running infinite loop

Below program will start incrementing the value from 1 to infinite.

class WhileLoopEx
{
   public static void  main(String args[])
   {
      int a=1;
      while(a > 0)
      {
        a++;
	  System.out.println("a : "+a);
      }
   }
}

Output