C++ while and do...while Loop

C++ while and do...while Loop

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,

  • 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

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

A 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
}

Check out these examples to learn more: