C++ while

last modified January 9, 2023

C++ while tutorial shows how to create loops in C++ with the while statement.

The while statement

The while statement is used to create a while loop. A while loop is a control flow statement executes code repeatedly based on the given boolean condition.

This is the general form of the while loop:

while (expression) {

statement(s);

}

The while keyword executes the statements inside the block enclosed by the curly brackets. The statements are executed each time the expression is evaluated to true.

C++ while example

The following example uses a while statement to calculate a sum.

simple.cpp

#include <iostream>

int main() {

int i = 0;
int sum = 0;

while (i <= 10) {

    sum += i;
    i++;
}

std::cout << sum << std::endl;

return 0;

}

We calculate the sum of 1..10 numbers.

The while loop has three parts: initialization, testing and updating. Each execution of the statement is called a cycle.

int i = 0;

We initiate the i variable. It is used as a counter.

while (i <= 10) { … }

The expression inside the brackets following the while keyword is the second phase: the testing. The statements in the body are executed until the expression is evaluated to false.

i++;

This is the last, third phase of the while loop: the updating. We increment the counter. Note that improper handling of the while loops may lead to endless cycles.

$ ./simple 55

C++ while - calculate factorial

The factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n.

n! = n * (n-1) * (n-2) * … * 1

This is the formula to calculate the factorial.

factorial.c

#include <iostream>

int main() {

int i = 10;
int factorial = 1;

while (i > 1) {

    factorial *= i;
    i--;
}

std::cout << factorial << std::endl;

return 0;

}

In the example, we use the while loop to calculate the 10! factorial.

$ ./factorial 3628800

C++ while - endless loop

The while (1) creates an endless loop. In order to terminate the loop, we use the break statement.

random.cpp

#include <random> #include <iostream>

using u32 = uint_least32_t; using engine = std::mt19937;

int main() {

std::random_device os_seed;
const u32 seed = os_seed();

engine generator(seed);
std::uniform_int_distribution<u32> distribute(1, 30);

while (1) {

    int r = distribute(generator);

    std::cout << r << " ";

    if (r == 22) {

        break;
    }
}

std::cout << std::endl;

return 0;

}

The example calculates a random value between 0..29. If it equals to 22, the loop is finished with the break statement.

$ ./random 25 28 9 9 18 6 20 2 5 15 7 11 22

C++ while - loop over array

The while statement can be used to traverse over an array.

loop_array.cpp

#include <iostream>

int main() {

int vals[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

int i, sum = 0;
size_t len = sizeof vals / sizeof vals[0];

while (i <= len) {

    sum += vals[i];
    i++;
}

std::cout << "The sum is: " << sum << std::endl;

return 0;

}

In the example, we have an array of integers. We go over the array with the while statement and calculate the sum of the values.

$ ./loop_array The sum is: 55

C++ do while example

The do while statement is a specific form of a while statement, where the block is executed before the condition. So the block is always executed at least once.

do_while.cpp

#include <iostream>

int main() {

int val, sum = 0;

do {

    std::cout << "Enter a number: ";
    std::cin >> val;
    sum += val;

} while(val != 0);

std::cout << "The sum is: " << sum << std::endl;

return 0;

}

The example asks the user to repeatedly enter a number. It calculates the sum of all those values. It terminates the loop when the user enters zero.

$ ./do_while Enter a number: 3 Enter a number: 2 Enter a number: 1 Enter a number: 0 The sum is: 6

This tutorial was dedicated to the while statement in C++.