Lære do-while Loop in Java | Exploring Loop Variations

Meet the do-while Loop

The do-while loop is a special kind of loop in Java that always runs its block of code at least once, no matter what. This is different from other loops like the while loop, which may not run at all if its condition is false at the start.

You use a do-while loop when you want to guarantee that a set of instructions runs before checking a condition. This is helpful for things like showing a menu to a user or asking for input at least one time.

Key features:

The code block runs once before the condition is checked;
The loop repeats as long as the condition stays true;
The condition is checked after the code block each time.

Here is the basic structure in pseudocode:

do {
    // Code to run at least once
} while (condition is true);

You can remember: do first, check later!

Task

Imagine you are building a program that asks for a user's age and ensures it is a valid number between 1 and 120. Normally, you would get input from the user, but here we simulate input with a fixed number.

The goal is to demonstrate how a do-while loop works: the program executes the input check at least once and continues looping until a valid age is provided.

Main.java


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package com.example;

public class Main {
    public static void main(String[] args) {
        int age = -5; // Simulated user input
        boolean valid;

        do {
            System.out.println("Checking age: " + age);

            if (age >= 1 && age <= 120) {
                valid = true;
                System.out.println("Valid age entered: " + age);
            } else {
                valid = false;
                System.out.println("Invalid age! Please enter a value between 1 and 120.");
                // Simulate the user entering a valid age next
                age = 25; 
            }
        } while (!valid);
    }
}

This program demonstrates how a do-while loop works by simulating age input validation. It ensures that the age is a positive number between 1 and 120. The loop executes at least once and continues until a valid age is provided.

The loop continues running as long as the condition evaluates to true after each execution of the loop body.

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