C++ Programming

while and do…while Loop in C++

In this tutorial, we will learn the use of while and do…while loops in C++ programming with the help of some examples.

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 of loops.

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,

  • 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

C++ while loop flowchart
Flowchart of C++ 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.

Iteration Variable i <= 5 Action
1st i = 1 true 1 is printed and i is increased to 2.
2nd i = 2 true 2 is printed and i is increased to 3.
3rd i = 3 true 3 is printed and i is increased to 4
4th i = 4 true 4 is printed and i is increased to 5.
5th i = 5 true 5 is printed and i is increased to 6.
6th i = 6 false The loop is terminated

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: 6
Enter a number: 12
Enter a number: 7
Enter a number: 0
Enter a number: -2

The 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

C++ do...while loop flowchart
Flowchart of C++ 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.

Iteration Variable i <= 5 Action
i = 1 not checked 1 is printed and i is increased to 2
1st i = 2 true 2 is printed and i is increased to 3
2nd i = 3 true 3 is printed and i is increased to 4
3rd i = 4 true 4 is printed and i is increased to 5
4th i = 5 true 5 is printed and i is increased to 6
5th i = 6 false The loop is terminated

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: 6
Enter a number: 12
Enter a number: 7
Enter a number: 0
Enter a number: -2

The 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: -6
The 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 loop
while(true) {
    // body of the loop
}

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

// infinite do...while loop

int 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

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

// This loop is iterated 5 times
for (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
}
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