8.4 Do-While Loops

Chapter Table of Contents

Section Table of Contents

Chapter Table of Contents Section Table of Contents

There is a variation of a while loop called a do-while loop. The only difference between the two is that the code inside of a do-while loop always runs at least once. This is the general format:


do { 
				// do stuff
} while (condition);

Notice that the condition is put at the end of the curly braces and that there is a semicolon at the end, unlike at the end of a for loop or while loop.

Here’s an example of how a do-while loop can be used:


import java.util.Scanner;

final int SECRET_NUMBER = 12;

System.out.println("Welcome to the guessing game!");
System.out.println("Guess the correct positive integer or -1 to quit.");

Scanner input = new Scanner(System.in);

int guess;
do {
     // prompt user for their guess
     System.out.print("Enter your guess: ");
     guess = input.nextInt();
} while (guess != SECRET_NUMBER && guess != -1);
        
if (guess == SECRET_NUMBER) { // user guessed correctly
	     System.out.println("Good job, you guessed right!");
} else { // user quit
      System.out.println("The secret number was " + SECRET_NUMBER);
}

Sample output 1:


Welcome to the guessing game!
Guess the correct positve integer or -1 to quit.
Enter your guess: 5
Enter your guess: 10
Enter your guess: -1
The secret number was 12

Sample output 2:


Welcome to the guessing game!
Guess the correct positve integer or -1 to quit.
Enter your guess: 34
Enter your guess: 52
Enter your guess: 12
Good job, you guessed right!

The program above is a simple guessing game. It prompts the user to enter guesses for a secret number (which is 12). It keeps asking the user to guess while their guess is incorrect and not equal to -1. (-1 is the sentinel value which the user uses to tell the program that they want to stop.)

Notice that I have to declare guess to be outside of the do-while block. If I declared it inside of the block, I would not be able to access the variable in the condition of the loop because the variable’s scope would only be within those curly braces.

💻 DoWhile.java

Previous Section

3️⃣

8.3 While Loops

Next Section

5️⃣

8.5 Break and Continue

⚖️

Copyright © 2021 Code 4 Tomorrow. All rights reserved. The code in this course is licensed under the MIT License. If you would like to use content from any of our courses, you must obtain our explicit written permission and provide credit. Please contact classes@code4tomorrow.org for inquiries.