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Introduction
The do-while
loop in Java is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. Unlike the while
loop, the do-while
loop guarantees that the code block is executed at least once before the condition is tested. This makes the do-while
loop particularly useful for scenarios where you need to ensure that the code block executes at least once.
Table of Contents
- What is a do-while Loop?
- Syntax of do-while Loop
- How do-while Loop Works
- Simple do-while Loop Example
- Infinite do-while Loop
- do-while Loop with a Break Statement
- do-while Loop with a Continue Statement
- Nested do-while Loop
- Using a do-while Loop for Input Validation
- Conclusion
What is a do-while Loop?
A do-while
loop repeatedly executes a block of code as long as the specified condition evaluates to true. The key difference from a while
loop is that the condition is evaluated after the loop body, ensuring that the loop body is executed at least once.
Syntax of do-while Loop
Syntax:
do { // body of loop } while (condition);
- condition: A boolean expression that is evaluated after each iteration of the loop.
- body of loop: The block of code that is executed at least once and as long as the condition is true.
How do-while Loop Works
- The loop body is executed.
- The condition is evaluated.
- If the condition is true, the loop body is executed again.
- Steps 2 and 3 are repeated until the condition becomes false.
- When the condition is false, the loop terminates, and control moves to the next statement after the loop.
Simple do-while Loop Example
Example:
public class SimpleDoWhileLoop { public static void main(String[] args) { int count = 1; do { System.out.println("Count: " + count); count++; } while (count <= 5); } }
Explanation: This loop prints the numbers from 1 to 5. The body of the loop is executed first, and then the condition count <= 5
is checked. Since the condition is true, the loop continues to execute until count
exceeds 5.
Infinite do-while Loop
An infinite loop occurs when the condition always evaluates to true.
Example:
public class InfiniteDoWhileLoop { public static void main(String[] args) { do { System.out.println("This is an infinite loop"); } while (true); } }
Explanation: This loop will print "This is an infinite loop" indefinitely because the condition true
never changes.
do-while Loop with a Break Statement
The break
statement can be used to exit the loop prematurely.
Example:
public class DoWhileWithBreak { public static void main(String[] args) { int count = 1; do { if (count == 5) { break; } System.out.println("Count: " + count); count++; } while (count <= 10); } }
Explanation: This loop prints numbers from 1 to 4. When count
equals 5, the break
statement exits the loop.
do-while Loop with a Continue Statement
The continue
statement skips the current iteration and moves to the next iteration of the loop.
Example:
public class DoWhileWithContinue { public static void main(String[] args) { int count = 0; do { count++; if (count % 2 == 0) { continue; } System.out.println("Odd number: " + count); } while (count < 10); } }
Explanation: This loop prints odd numbers from 1 to 9. The continue
statement skips the even numbers.
Nested do-while Loop
A nested do-while
loop is a do-while
loop inside another do-while
loop, useful for iterating over multidimensional structures.
Example:
public class NestedDoWhileLoop { public static void main(String[] args) { int i = 1; do { int j = 1; do { System.out.println("i: " + i + ", j: " + j); j++; } while (j <= 3); i++; } while (i <= 3); } }
Explanation: This loop prints pairs of i
and j
values, iterating over all combinations of i
and j
from 1 to 3.
Using a do-while Loop for Input Validation
The do-while
loop is often used for validating user input.
Example:
import java.util.Scanner; public class InputValidation { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); int number; do { System.out.print("Enter a positive number: "); number = scanner.nextInt(); if (number <= 0) { System.out.println("Invalid input. Please try again."); } } while (number <= 0); System.out.println("You entered: " + number); scanner.close(); } }
Explanation: This loop repeatedly prompts the user to enter a positive number until a valid input is provided.
Conclusion
The do-while
loop in Java is a powerful control flow statement for performing repeated tasks based on a condition. Unlike the while
loop, the do-while
loop guarantees that the code block is executed at least once. Understanding how to use the do-while
loop effectively, including its variations with break
and continue
statements, as well as nested loops, is essential for writing robust and efficient Java programs.
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