C++ while and do...while Loop (With Examples) (2024)

In computer programming, loops are used to repeat a block of code.

For example, let's say we want to show a message 100 times. Then instead of writing the print statement 100 times, we can use a loop.

That was just a simple example;we can achieve much more efficiency and sophistication in our programs by making effective use ofloops.

There are 3 types of loops in C++.

1. for loop
2. while loop
3. do...while loop

In the previous tutorial, we learned about the C++ for loop. Here, we are going to learn about while and do...while loops.

C++ while Loop

The syntax of the while loop is:

while (condition) { // body of the loop}

Here,

• A while loop evaluates the condition
• If the condition evaluates to true, the code inside the while loop is executed.
• The condition is evaluated again.
• This process continues until the condition is false.
• When the condition evaluates to false, the loop terminates.

To learn more about the conditions, visit C++ Relational and Logical Operators.

Flowchart of while Loop

Example 1: Display Numbers from 1 to 5

// C++ Program to print numbers from 1 to 5#include <iostream>using namespace std;int main() { int i = 1; // while loop from 1 to 5 while (i <= 5) { cout << i << " "; ++i; } return 0;}

Output

1 2 3 4 5

Here is how the program works.

Example 2: Sum of Positive Numbers Only

// program to find the sum of positive numbers// if the user enters a negative number, the loop ends// the negative number entered is not added to the sum#include <iostream>using namespace std;int main() { int number; int sum = 0; // take input from the user cout << "Enter a number: "; cin >> number; while (number >= 0) { // add all positive numbers sum += number; // take input again if the number is positive cout << "Enter a number: "; cin >> number; } // display the sum cout << "\nThe sum is " << sum << endl; return 0;}

Output

Enter a number: 6Enter a number: 12Enter a number: 7Enter a number: 0Enter a number: -2The sum is 25

In this program, the user is prompted to enter a number, which is stored in the variable number.

In order to store the sum of the numbers, we declare a variable sum and initialize it to the value of 0.

The while loop continues until the user enters a negative number. During each iteration, the number entered by the user is added to the sum variable.

When the user enters a negative number, the loop terminates. Finally, the total sum is displayed.

C++ do...while Loop

The do...while loop is a variant of the while loop with one important difference: the body of do...while loop is executed once before the condition is checked.

Its syntax is:

do { // body of loop;}while (condition);

Here,

• The body of the loop is executed at first. Then the condition is evaluated.
• If the condition evaluates to true, the body of the loop inside the do statement is executed again.
• The condition is evaluated once again.
• If the condition evaluates to true, the body of the loop inside the do statement is executed again.
• This process continues until the condition evaluates to false. Then the loop stops.

Flowchart of do...while Loop

Example 3: Display Numbers from 1 to 5

// C++ Program to print numbers from 1 to 5#include <iostream>using namespace std;int main() { int i = 1; // do...while loop from 1 to 5 do { cout << i << " "; ++i; } while (i <= 5); return 0;}

Output

1 2 3 4 5

Here is how the program works.

Example 4: Sum of Positive Numbers Only

// program to find the sum of positive numbers// If the user enters a negative number, the loop ends// the negative number entered is not added to the sum#include <iostream>using namespace std;int main() { int number = 0; int sum = 0; do { sum += number; // take input from the user cout << "Enter a number: "; cin >> number; } while (number >= 0); // display the sum cout << "\nThe sum is " << sum << endl; return 0;}

Output 1

Enter a number: 6Enter a number: 12Enter a number: 7Enter a number: 0Enter a number: -2The sum is 25

Here, the do...while loop continues until the user enters a negative number. When the number is negative, the loop terminates; the negative number is not added to the sum variable.

Output 2

Enter a number: -6The sum is 0.

The body of the do...while loop runs only once if the user enters a negative number.

Infinite while loop

If the condition of a loop is always true, the loop runs for infinite times (until the memory is full). For example,

// infinite while loopwhile(true) { // body of the loop}

Here is an example of an infinite do...while loop.

// infinite do...while loopint count = 1;do { // body of loop} while(count == 1);

In the above programs, the condition is always true. Hence, the loop body will run for infinite times.

for vs while loops

A for loop is usually used when the number of iterations is known. For example,

// This loop is iterated 5 timesfor (int i = 1; i <=5; ++i) { // body of the loop}

Here, we know that the for-loop will be executed 5 times.

However, while and do...while loops are usually used when the number of iterations is unknown. For example,

while (condition) { // body of the loop}

Also Read:

• C++ Program to Display Fibonacci Series
• C++ Program to Find GCD
• C++ Program to Find LCM