# What is a loop?

When programming, you will encounter blocks of code that are to be repeated over and over again until a specific requirement becomes false. Hence, this block of code will run in a loop. 

One specific type of loop is a **while loop**. This form of a loop continues to run as long as a specific condition returns *true*. A `while loop` follows the syntax given below:

class HelloWorld {
    public static void main( String args[] ) {
        
        while(condition){
          //body of loop
        }
    }
}

# Syntax

Look at the code above from *lines 4 to 6* to understand the syntax of a while loop in Java:

* A `while` keyword is followed by round brackets, `()`
* Within these brackets, the `condition` is written 
* This `condition` is checked each time a loop is run. If the condition returns `true` then the *loop body* executes, otherwise the loop *breaks.*

The illustration below shows the logical route that a while loop follows:

# Example

Now that we know what syntax the while loop follows and how to write it, let's look at a few coding examples.

**1. Starting `while` with a false condition** 

The example below shows what happens when you execute a while loop when the condition is false such that the loop does not execute even once:

class HelloWorld {
    public static void main( String args[] ) {
        int i=10;
        while(i<5){
          System.out.println("The value of i: "+ i);
        }
    }
}

**2. Executing a `while` loop**

The example below shows how the loop runs until the conditional statement returns false. For each iteration of the loop, the value of `i` is doubled. When the value becomes *greater than 30*, the loop should terminate.

class HelloWorld {
    public static void main( String args[] ) {
        int i=1;
        while(i<30){
          System.out.println("The value of i is "+ i);
          i=i*2;
        }
    }
}

**3. Using `while` to iterate over an array**

We can even use while loops to iterate over an array. 

* First we define an `array` and find out its length, `len`, using the `length` property of arrays. 

* Then we define an `index` variable to iterate over the `array`.
 
* The while loop, from *lines 6 to 9*, is run till `index` becomes equal to `len` indicating that the entire array is traversed. 

* In each iteration, the loop condition checks if the `index` is less than `len`. If the result of the expression is true, i.e. `index` is less than `len`, then the value of the array at `index`  is printed and `index` is incremented

The example below shows how it can be done:

class HelloWorld {
    public static void main( String args[] ) {
       int array [] = {5,4,3,2,1};
       int len = array.length;
       int index = 0;
       while(index<len){
         System.out.println(array[index]);
         index = index + 1;
       }
    }
}

How do you execute a 'while' loop in Java?

What is a loop?

Syntax

Example