Do looping

Java do-while Loop

The Java do-while loop is used to iterate a portion of the program repeatedly until the specified requirement is true.

If the loop needs to be executed at least once and the number of iterations is uncertain, a do-while loop is recommended.
An exit control loop is a type of do-while loop in Java. Therefore, in contrast to the loop and for the loop, the do-while checks the status at the end of the loop body.
The Java do-while loop is executed at least once because the condition is checked after the loop body.

Syntax of do-while loop:

Java do while loop syntax is as follows:

The expression for the do-while loop must return a boolean value, otherwise, it will throw compile-time error.

Here’s a more detailed explanation of each part of the “do-while” loop:

1. do: The “do-while” loop is initiated by this keyword. It is used to indicate that the block of code following it should be executed at least once, regardless of the condition.
2. {statement}: This represents the body of the loop, which contains the code that will be executed repeatedly as long as the condition is true.
3. While  (condition): This is the loop continuation condition. The loop will continue as long as this condition evaluates to true. After each iteration of the loop body, the condition is checked again. If it is still true, the loop will execute the body again. The loop will end and the program will proceed with the code that follows the loop if the condition turns false.

The “do-while” loop can be handy when you want to execute a block of code at least once, and then you want to repeat it based on a certain condition.

 Example:

public class LoopExample {

    public static void main(String[] args) {
        int i = 0;
        do {
            System.out.println(i);
            i++;
        } while (i < 5);

    }

}

Output:

Explanation:

1. int i = 0; Here, you are declaring a variable i and initializing it with the value 0.
2. do { … } while (i < 5);This is a “do-while” loop. The code within the block will be executed repeatedly as long as the condition i < 5 is true.
3. System.out.println(i); This line of code prints the current value of i to the console.
4. i++;This line increments the value of i by 1 in each iteration of the loop. It’s a shorthand for i = i + 1.

Now, let’s walk through the iterations of the loop:

The given do-while loop iterates through a sequence of numbers as follows:

Iteration 1:

• i is 0.
• Print the value of i (which is 0).
• Increment i by 1.

Iteration 2:

• i is 1.
• Print the value of i (which is 1).
• Increment i by 1.

Iteration 3:

• i is 2.
• Print the value of i (which is 2).
• Increment i by 1.

Iteration 4:

• i is 3.
• Print the value of i (which is 3).
• Increment i by 1.

Iteration 5:

• i is 4.
• Print the value of i (which is 4).
• Increment i by 1.

Iteration 6:

• i is 5.
• The loop terminates because the condition i < 5 is no longer satisfied.

The loop runs a total of 6 iterations, printing the numbers 0 through 4 in sequential order. It stops after the sixth iteration because the value of i becomes 5, which makes the condition i < 5 false, causing the loop to exit.

Another Example of the do-while loop :

In the below example, we print integer values from 1 to 10. Unlike the for loop, we separately need to initialize and Decrement the variable used in the condition . Otherwise, the loop will execute infinitely.

public class LoopExample {

    public static void main(String[] args) {
        int i = 10;
        do {
            System.out.println(i);
            i--;
        } while (i > 1);
    }

}

Output:

Explanation:

1. int i = 10; Here, you are declaring a variable i and initializing it with the value 10.
2. do { … } while (i > 1); This is a “do-while” loop. The code within the block will be executed repeatedly if the condition i > 1 is true.
3. System.out.println(i); This line of code prints the current value of i to the console.
4. i–; This line decrements the value of i by 1 in each iteration of the loop. It’s a shorthand for i = i – 1.

Now, let’s walk through the iterations of the loop:

1. The provided do-while loop iterates through a sequence of numbers as follows:Iteration 1:
   • i is 10.
   • Print the value of i (which is 10).
   • Decrement i by 1.

   Iteration 2:

   • i is 9.
   • Print the value of i (which is 9).
   • Decrement i by 1.

   Iteration 3:

   • i is 8.
   • Print the value of i (which is 8).
   • Decrement i by 1.

   Iteration 4:

   • i is 7.
   • Print the value of i (which is 7).
   • Decrement i by 1.

   Iteration 5:

   • i is 6.
   • Print the value of i (which is 6).
   • Decrement i by 1.

   Iteration 6:

   • i is 5.
   • Print the value of i (which is 5).
   • Decrement i by 1.

   Iteration 7:

   • i is 4.
   • Print the value of i (which is 4).
   • Decrement i by 1.

   Iteration 8:

   • i is 3.
   • Print the value of i (which is 3).
   • Decrement i by 1.

   Iteration 9:

   • i is 2.
   • The loop terminates because the condition i > 1 is no longer satisfied.

   The loop runs a total of 9 iterations, printing the numbers 10 through 2 in sequential order. It stops after the ninth iteration because the value of i becomes 2, which makes the condition i > 1 false, causing the loop to exit

Iterating array using do-while loop

public class doWhileLoopingExample  {
    public static void main(String[] args) {
        int arr[] = { 3, 21, 35, 6 };

     // i starts with 0 as array index starts with 0
        int i = 0;
        do {
            System.out.println(arr[i]);

         i++;
        } while (i < 4);