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3. C++ while and do...while Loop

Loops are used in programming to repeat a specific block of code. In this article, you will learn to create while and do...while loops in C++ programming.


In computer programming, loop repeats a certain block of code until some end condition is met.
There are 3 type of loops in C++ Programming:

C++ while Loop

The syntax of a while loop is:
while (testExpression) 
{
     // codes  
}

How while loop works?

  • The while loop evaluates the test expression.
  • If the test expression is true, codes inside the body of while loop is evaluated.
  • Then, the test expression is evaluated again. This process goes on until the test expression is false.
  • When the test expression is false, while loop is terminated.

Flowchart of while Loop

Example 1: C++ while Loop

// C++ Program to compute factorial of a number
// Factorial of n = 1*2*3...*n

#include <iostream>
using namespace std;

int main() 
{
    int number, i = 1, factorial = 1;

    cout << "Enter a positive integer: ";
    cin >> number;
    
    while ( i <= number) {
        factorial *= i;      //factorial = factorial * i;
        ++i;
    }

    cout<<"Factorial of "<< number <<" = "<< factorial;
    return 0;
}
Output
Enter a positive integer: 4
Factorial of 4 = 24
In this program, user is asked to enter a positive integer which is stored in variable number. Let's suppose, user entered 4.

Then, the while loop starts executing the code. Here's how while loop works:
  1. Initially, i = 1, test expression i <= number is true and factorial becomes 1.
  2. Variable i is updated to 2, test expression is true, factorial becomes 2.
  3. Variable i is updated to 3, test expression is true, factorial becomes 6.
  4. Variable i is updated to 4, test expression is true, factorial becomes 24.
  5. Variable i is updated to 5, test expression is false and while loop is terminated.

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 test expression is checked.
The syntax of do..while loop is:
do {
   // codes;
}
while (testExpression);

How do...while loop works?

  • The codes inside the body of loop is executed at least once. Then, only the test expression is checked.
  • If the test expression is true, the body of loop is executed. This process continues until the test expression becomes false.
  • When the test expression is false, do...while loop is terminated.

Flowchart of do...while Loop

Example 2: C++ do...while Loop

// C++ program to add numbers until user enters 0

#include <iostream>
using namespace std;

int main() 
{
    float number, sum = 0.0;
    
    do {
        cout<<"Enter a number: ";
        cin>>number;
        sum += number;
    }
    while(number != 0.0);

    cout<<"Total sum = "<<sum;
    
    return 0;
}
Output
Enter a number: 2
Enter a number: 3
Enter a number: 4
Enter a number: -4
Enter a number: 2
Enter a number: 4.4
Enter a number: 2
Enter a number: 0

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