Java Do/While Loop -

Java Do/While Loop

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The do-while loop in Java is similar to the while loop, but with a key difference: the do-while loop guarantees that the code block will execute at least once, even if the condition is initially false. This is because the condition is evaluated after the code block is executed.

Syntax of Do/While Loop

do {
    // Code block to be executed
} while (condition);
  • do: The block of code inside the do statement is executed first, before the condition is tested.
  • while (condition): After executing the block, the condition is checked. If it’s true, the code block will be executed again. If false, the loop terminates.

Basic Example of Do/While Loop

Here’s a simple example that prints the numbers from 1 to 5 using the do-while loop:

public class DoWhileExample {
    public static void main(String[] args) {
        int i = 1;

        do {
            System.out.println(i);
            i++;
        } while (i <= 5);
    }
}

Output:

1
2
3
4
5

In this example, the loop will print the numbers from 1 to 5. The condition i <= 5 is checked after the code block is executed, ensuring that the code runs at least once.

How the Do/While Loop Works:

  1. Code Execution: The code inside the do block is executed first.
  2. Condition Check: After the code execution, the condition in the while statement is checked.
  3. If the condition is true, the loop repeats.
  4. If the condition is false, the loop stops.

Difference Between While and Do/While Loop

The main difference between a while loop and a do-while loop is when the condition is checked:

  • In a while loop, the condition is checked before executing the code.
  • In a do-while loop, the condition is checked after the code block is executed, ensuring the code runs at least once.

Example of While vs Do/While Loop

// While loop example
int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}

// Do-While loop example
int j = 1;
do {
    System.out.println(j);
    j++;
} while (j <= 5);

Both loops will output the same result, but the do-while loop guarantees that the code executes once even if the condition is false.

Infinite Do/While Loop

If the condition in a do-while loop is always true, it will create an infinite loop.

public class InfiniteDoWhile {
    public static void main(String[] args) {
        do {
            System.out.println("This will print forever!");
        } while (true);
    }
}

Output:

This will print forever!
This will print forever!
...

To stop the loop, you would need to manually interrupt the program, such as by using Ctrl + C in the terminal.

Using break and continue in Do/While Loop

You can also use break and continue in a do-while loop to control its execution:

  • break: Exits the loop immediately, regardless of the condition.
  • continue: Skips the current iteration and moves to the next iteration.

Example with break

public class BreakExample {
    public static void main(String[] args) {
        int i = 1;

        do {
            if (i == 3) {
                break;  // Exits the loop when i is 3
            }
            System.out.println(i);
            i++;
        } while (i <= 5);
    }
}

Output:

1
2

Example with continue

public class ContinueExample {
    public static void main(String[] args) {
        int i = 1;

        do {
            i++;
            if (i == 3) {
                continue;  // Skips printing 3
            }
            System.out.println(i);
        } while (i <= 5);
    }
}

Output:

2
4
5

Real-Life Example: Input Validation with Do/While Loop

A practical use of the do-while loop is to repeatedly ask for user input until it meets a certain condition, such as ensuring that the input is a positive number.

import java.util.Scanner;

public class InputValidation {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int userInput;

        do {
            System.out.println("Enter a positive number:");
            userInput = sc.nextInt();
        } while (userInput <= 0);

        System.out.println("You entered a valid positive number: " + userInput);
        sc.close();
    }
}

Output:

Enter a positive number:
-5
Enter a positive number:
0
Enter a positive number:
10
You entered a valid positive number: 10

In this example, the loop keeps asking the user for input until a positive number is provided.

Key Points to Remember About Do/While Loop

  • The do-while loop guarantees that the block of code runs at least once, even if the condition is false initially.
  • It is ideal for situations where you need to ensure the code executes before checking the condition.
  • Be cautious of infinite loops, which occur when the condition always evaluates to true.
  • Use break to exit the loop prematurely and continue to skip an iteration.

The do-while loop is a useful tool for situations where the code must be executed at least once, and its flexibility with condition checking makes it a valuable part of your Java programming toolkit.

For more insights and practical coding tips, visit Master Coding Science at mastercodingscience.com.

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