do while

Definition

The do while statement is used to create a loop. It is similar to the while loop except that the test condition is evaluated at the end of the loop. Therefore, a do while loop will always execute at least once.

Syntax

do while

A simple do while statement has the following syntax:

• JavaScript
• Java
• Python

do {
    block;
} while ( condition );

do {
    block;
} while ( condition );

JavaScript and Java syntax.

• The condition is an expression that is evaluated at the end of each pass through the loop, and which evaluates to either true or false.
• The loop will stop executing when the condition becomes false.
• The block is a block of code that gets executed at least once and then each time the condition evaluates to true.

Examples

Example 1: Quiz Question

For this example we will check if a user understands a concept. We can prompt the user for an answer at least once and keep prompting until the user enters the correct answer. The do while loop can be used to achieve this.

1. The do while loop will ask the user to enter an answer (a, b or c) and check if the answer is correct (b). If the answer is not correct, the program loop will repeat and the user will be asked for an answer.
   
   However, the code does not recognise an upper case character 'B' as correct.
• JavaScript
• Java
• Python

var answer; 
do {
    answer = prompt("Loops at least once:  a) while, b) do while, c) for","");
} while ( answer != 'b' );     

alert("Correct! The do while loops at least once.");

JavaScript Note: The users answer is entered in a string text field so we check against a string in the do while loop condition.

Scanner input = new Scanner(System.in);
  
char answer;  
do {
    System.out.println("Loops at least once:  a) while, b) do while, c) for");
    System.out.println("Enter a, b or c");
    answer = input.next().charAt(0); 
} while (answer != 'b');  

System.out.println("Correct! The do while loops at least once.");  

Java Notes: Remember to import the Scanner at the start of your Java program (not shown in this example - import java.util.Scanner). The Scanner allows us to create an object to enable keyboard input. Here, the object is named input:

•    Scanner input = new Scanner(System.in);

The input object allows access to the next() method to get the next word in the input. The charAt(0) function returns the first character in that string:

•     answer = input.next().charAt(0);

Python Note: The do while loop is not supported by Python.

2. We now ensure the program is able to deal with both upper case and lower case input. The input value is converted to lower case prior to testing and will accept both 'b' and 'B' input as the correct answer.
• JavaScript
• Java
• Python

var answer; 
do {
    answer = prompt("Loops at least once:  a) while, b) do while, c) for"," ");
    answer = answer.toLowerCase();   
} while ( answer != 'b' );      

alert("Correct! The do while loops at least once.");

JavaScript Note: In line 4 the toLowerCase() method returns the string with any upper case letters converted to lower case letters. We store the returned string back into the variable answer.

Scanner input = new Scanner(System.in);
  
char answer;  
do {
    System.out.println("Loops at least once:  a) while, b) do while, c) for");
    System.out.println("Enter a, b or c");
    answer = input.next().charAt(0); 
    answer = Character.toLowerCase(answer);
} while (answer != 'b');  

System.out.println("Correct! The do while loops at least once."); 

Java Note: In line 3 we declare the answer variable as a char. In line 7 the program returns the lower case equivalent of the character, if any, or the character itself and stores it back in answer.

Python Note: The do while loop is not supported by Python.

Example 2: Guess a Secret Number

In this second example we will use the do while loop to create a simple game that prompts a user to guess a secret number between 1 and 15. The user enters guesses until their input matches the secret number.

1. We create a variable secretNumber and give it the integer value 10. A variable userGuess is used to store the guess entered by the user and variable guesses will be used to track how many guesses the user takes.
   
   Each time through the loop we get the next guess from the user and we update the number of guesses by one. We will repeat the loop block until the number provided by user matches the value stored in secretNumber.
   
   When the number is guessed we will exit the loop and let the user know how many guesses they took to find the secret number.
• JavaScript
• Java
• Python

var secretNumber = 10;
var guesses = 0; 
var userGuess;

do {
    userGuess = prompt("Enter a number between 1 and 15","");
    userGuess = parseInt(userGuess);
    guesses++;  
} while (userGuess != secretNumber);

alert("Guessed in " + guesses + " attempt(s).");

JavaScript Note: As the users guess is entered in a text field, the data type will be a string. To convert it to an integer number use the parseInt() function.

Scanner input = new Scanner(System.in);
  
int secretNumber = 10;
int guesses = 0; 
int userGuess;

System.out.println("Guess the Number Game");

do {
    System.out.println("Enter a number between 1 and 15");
    userGuess  = input.nextInt();   
    guesses++; 
} while (userGuess != secretNumber);

System.out.println("Guessed in " + guesses + " attempt(s).");

Java Notes:

• Java requires that the variable type is defined before use.
• The nextInt() method gets the integer number entered by the user.

Python Note: The do while is not supported by Python.

2. Now we will extend the program to give the a hint (higher or lower) based on the users last input.
   
   We create an empty string variable hint which is updated within the loop with an appropriate message only if the users guess is higher or lower than the secret number. The value in this variable is concatenated (joined) to the message displayed when the user is asked for a guess.
• JavaScript
• Java
• Python

var secretNumber = 10;
var guesses = 0; 
var hint = ''; 
var userGuess;	  
	  
do {
    userGuess = prompt("Enter a number between 1 and 15" + hint," ");
    userGuess = parseInt(userGuess);
    guesses++;  
    if (userGuess > secretNumber) {
        hint = ' (HINT: less than ' + userGuess + ')';
    } else if (userGuess < secretNumber) {
        hint = ' (HINT: greater than ' + userGuess + ')';
    }
} while (userGuess != secretNumber);

alert("Guessed in " + guesses + " attempt(s).");

Scanner input = new Scanner(System.in);
  
int secretNumber = 10;
int guesses = 0; s
int userGuess;
String hint = ""; 
	  
System.out.println("Guess the Number Game"); 
do {
    System.out.println("Enter a number between 1 and 15" + hint + "");
	userGuess  = input.nextInt();   
    guesses++;  
    if (userGuess > secretNumber) {
        hint = " (HINT: less than " + userGuess + ")";
    } else if (userGuess < secretNumber) {
        hint = " (HINT: greater than " + userGuess + ")";
    }
} while (userGuess != secretNumber);

System.out.println("Guessed in " + guesses + " attempt(s).");

Python Note: The switch case is not supported by Python.