C++ while and do…while Loop

C++ while and do…while Loop

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

In computer programming, loops are used to rehash a square of code.

For instance, suppose we need to show a message 100 times. At that point as opposed to writing the print proclamation multiple times, we can use a loop.

That was only a straightforward model; we can accomplish substantially more productivity and sophistication in our programs by using loops.

There are 3 sorts of circles in C++.

for loop
while loop
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.

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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.

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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

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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 provoked to enter a number, which is put away in the variable number.

So as to store the whole of the numbers, we pronounce a variable sum and instate it to the value of 0.

The while loop proceeds until the client enters a negative number. During every emphasis, the number entered by the user is added to the sum variable.

At the point when the user enters a negative number, the loop terminates. Finally, the total sum is displayed.

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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 from the start. At that point, the condition is evaluated.

If the condition evaluates to true, the body of the loop inside the do statement is executed once more.

The condition is evaluated once again.

If the condition evaluates to true, the body of the loops inside the do articulation is executed once more.

This procedure proceeds until the condition evaluates to false. then the loop stops.

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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

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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 proceeds until the user enters a negative number. At the point when the number is negative, the loop terminates; the negative number isn’t 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.

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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.

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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 realize that the for-loop will be executed 5 times.

However, while and do…while loops are normally utilized when the quantity of emphasess is obscure. For instance,

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

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Please feel free to give your comment if you face any difficulty here.

C++ Flow Control