Factorial calculation

Write a C++ program that calculates the factorial of a given number. Before calculating the factorial, use a do-while loop to make sure that the user inputs a non-negative integer.

Then, use a for loop to calculate the factorial of the given number.

Answer

#include <iostream>

int main() {

    // Get input from user until it is non-negative
    int input;
    do {
        std::cout << "Enter a non-negative integer: ";
        std::cin >> input;
    } while (input < 0);

    // Calculate factorial by multiplying all numbers from 1 to input together
    int factorial = 1;
    for (int i = 1; i <= input; ++i) {
        factorial *= i;
    }

    // Print result
    std::cout << "Factorial of " << input << " = " << factorial << "\n";
}

Integers aren't large enough

ints only take up 4 bytes of memory. Because of this the highest value they can store is 2,147,483,647. The factorial of 13 is 6,227,020,800.

Because of this we need to use a larger datatype to store the factorial.

unsigned long long factorial = 1;

Depending on compiler and system architecture; a long is either 4 or 8 bytes. The long long type ensures at least 8 bytes, offering a larger range of values than a single long.

The unsigned keyword specifies that a variable can only hold positive numbers (or 0). This effectively doubles the positive range.

For refference: unsigned long long has a range from 0 to 18,446,744,073,709,551,615. long long has a range from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807.