Lesson 18 of 31

5 mins read

Do-While Loops

In Plain English

The `do-while` loop is a post-test loop. It executes its code block FIRST, and only then tests the condition at the bottom. This guarantees that the loop body will ALWAYS execute at least once, regardless of whether the initial condition is true or false.

The Mental Model

"A do-while loop is like a restaurant that serves you the appetizer first and checks your membership card after. You are guaranteed at least one bite."

Deep Dive: How It Works

Syntax: `do { /* statements */ } while (condition);`

Notice the semicolon `;` required at the end after `while(condition);`.

Commonly used for interactive menu systems and prompt re-tries where you must prompt the user at least once before validating input.

Syntax Blueprint

int x = 10;
do {
    System.out.println(x);
    x++;
} while (x < 5);

Prints 10 once, even though 10 < 5 is false right from the start.

Core Rules to Remember

Guaranteed Single Run: Always runs at least 1 time.

Trailing Semicolon: Must end with while (condition);

Interactive Menu Pattern: Perfect for prompting user before checking exit condition.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting the semicolon after while (condition);

Why it happens: Unlike standard while loops, do-while requires a semicolon after the closing parenthesis.

How to fix: Always write while (condition); with a final semicolon.

Pro Tip: Use do-while when the data you need to test in the condition is created inside the loop body itself.

Live Interactive Example

Hit Run Code to see it live

Do-While Guaranteed Run

Java 13

Output Console

Click "Run Code" to view the rendered output.

How it works: Executes attempt 1, increments to 2 (2 <= 2 is true), executes attempt 2, increments to 3, and stops.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Write a do-while loop

Initialize int step = 1; write a do-while loop that prints "Step " + step while step <= 3.
Remember to increment step inside the loop.

Sandbox Output

Click "Run & Check" to test your solution.

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