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While and do-while Loops

While and do-while Loops

Learn about the while and the do-while loop constructs in C++.

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Several programming tasks consist of executing the same code multiple times. Tasks like moving an object across the screen, iterating through a list of items, searching through data, and performing mathematical operations on a stream of numbers all consist of a block of code being executed repeatedly till a termination condition is met. C++ allows executing the same code several times using two different loop constructs:

  • The while loop and its variation, the do-while loop

  • The for loop

The while Loop

The while loop is really the only necessary repetition construct. The for loop and the do - while loop, coming up shortly, can be replicated using a while loop.

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while ( condition ) 
{
    //body
}

Note: A while statement doesn't use a semicolon at its end. It's because the semicolon is used to terminate statements, and the while statement itself is a control structure that needs to be followed by a block of code to execute as long as the condition is true.

A while loop continues to run as long as the condition inside the parentheses is true. If the condition is true, the code inside the curly braces {} is executed. After executing the last line inside the braces, the program checks the condition again. If it is still true, the code inside the curly braces runs again. This cycle repeats as long as the condition remains true. Once the condition becomes false, the while loop stops, and the program proceeds with the code following the closing curly brace }.

Coding example

Let’s look at a simple while loop that prints the integers from 1 to 5.

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C++
#include <iostream>
using namespace std;
int main() {
    int i = 1; // Initialize the counter variable
    while (i <= 5) 
    { 
        cout << " The value of i is:" << i << endl; 
        i = i + 1; // Increment the variable i by 1
    }
    return 0; // End of the program
}

Explanation

Let’s look at the line-by-line explanation of the above program.

  • Line 1: We include the <iostream> header to use input and output functions.

  • Line 2: We use using namespace std; so we don’t have to write std:: before every standard library object.

  • Line 4: The main function is defined, which is the entry point of the program.

  • Line 5: We declare an integer variable i and initialize it with the value 1.

  • Line 7: The while loop begins, checking the condition i <= 5. As long as this condition is true, the loop will execute the code inside the curly braces {} of the while loop.

  • Line 9: Within the loop, we print the current value of i followed by a newline.

  • Line 10: Still, within the loop, we increment the value of i by 1.

  • Line 11: The loop ends when the condition i <= 5 becomes false. The program then continues to the next statement.

  • Line 13: The return 0; statement ends the main function and the program. Returning 0 typically indicates that the program has been executed successfully.

Workflow

Let’s try to understand the flow of the program using the illustration below:

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Infinite loops

Let’s assess your understanding of while loops with the quiz below.

1.

Consider the following code:

#include <iostream>
using namespace std;

int main() {
    int i = 1;
    while (i <= 5) {
        cout << i << endl;
    }
    i = i + 1;
    return 0;
}

What would be the output of this code? Try running this program in the above widget and see what happens.

Show Answer
Q1 / Q1
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The above piece of code demonstrates an infinite loop. An infinite loop is one that either doesn’t have a termination condition, is structured in a manner, or acts on input data such that the termination condition is never met. Because of this, the loop continues to run until the program crashes or is forcefully exited.

Note: A common beginner's mistake is to forget to add code that updates the loop variable such that the loop condition eventually evaluates to false. Another common mistake is to add the line of code that updates the loop variable outside of the loop's body because of which the control never reaches that point.

The do-while loop

The do-while loop is identical to the while loop, but instead of checking the conditional statement before the loop starts, it checks the condition after the first run and then continues on to another iteration. Here’s the syntax of the do-while loop:

do {
    // body
} while (condition);

As you can see, it will run the loop at least once before checking the condition.

Note: The do-while loop does have a semicolon at the end. Infinite loops can still occur in do-while loops, so exercise the same caution as you would with while loops. Its usefulness is more limited, so use this only when necessary.

Coding example

The playground below shows how to implement the above example using a do-while loop instead.

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C++
#include <iostream>
using namespace std;
int main() {
    int i = 1;
    do {
        cout << "Value of i is: " << i << endl;
        i = i + 1;
    } while (i <= 5);  // the condition is being checked after the first run
    return 0;
}

Explanation

Let’s have a look at the line-by-line explanation of the above program.

  • Line 1: We include the <iostream> header to use input and output functions.

  • Line 2: We use the using namespace std; statement so we don’t have to write std:: before every standard library object.

  • Line 4: The main function is defined, which is the entry point of the program.

  • Line 5: We declare an integer variable i and initialize it with the value 1.

  • Line 6: The do block begins, printing the value of i and then incrementing its value by 1.

  • Line 7: We print the current value of i followed by a newline.

  • Line 8: Still within the do block, we increment the value of i by 1.

  • Line 9: The while statement checks if the value of i is less than or equal to 5. The loop continues to run as long as the condition is true.

  • Line 10: The return 0; statement ends the main function and the program. Returning 0 typically indicates that the program has been executed successfully.

Scenario

We have looked at the do-while loop, but where is it helpful? Let’s consider a scenario where we want to take an integer input from the user until the user has entered a positive number. In this case, we will use a do-while loop as we have to run the loop at least once because we want input from the user at least once. This loop will continue running until the user enters a positive number. Let’s have a look at the code:

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#include <iostream>
using namespace std;
int main() {
    int number;
    do {
        cout << "Enter a positive number: ";
        cin >> number;
    } while (number <= 0); // the condition is being checked after the first run
    cout << "You entered a positive number: " << number << endl;
    return 0;
}

As you can see, the above code will continue taking input from the user unless the user enters a positive number.

Exercise

Test your understanding by changing the above code such that instead of printing the numbers from 1 to 5, it prints the number from 5 to 1.

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C++
#include <iostream>
using namespace std;
int main() {
    
    // Write your code here
    return 0;
}

If you’re stuck, click the “Show Solution” button.