Question

Half an hour ago, I made a simple factorial calculator that takes non-zero integers as inputs. After testing it for some values, I noticed that it only work properly until 12!.

I haven’t been programming for some months now, and still am a beginner to be honest. I decided to use recursion so I could get back in to “programming mode” faster (my preference).

I checked and modified it for almost an hour now. I don’t really know what is wrong with my algorithm.

This is my code:

#include <iostream>

using namespace std;

int factorial(int);

int main()
{
    int usrInput = 0;   //initialize input variable
    cout << "Please input the factorial you want to calculate: ";
    cin >> usrInput;
    while(usrInput < 1)
    {
        cout << "Please input a valid number: ";
        cin >> usrInput;
    } //end while
    cout << "\nAnswer: " << factorial(usrInput) << '\n';
    return 0;
}

int factorial(int n)
{
    int product = n;
    if(n < 2)
        product = 1;
    else
    {
        product *= factorial(n-1);
    cout << "\n" << product; //debugging line
    } //end if else
    return product;
}

Answer 1

You exceed limits of int. 13! = 6227020800, int only covers -2147483648 .. 2147483647. Use bigger type (eg. __int64), double (but you’ll lose precision) or implement (or use) big number library.