C++ factorial program give infinity for any number greater than 2

Question

So it's 5:00 A.M. where I am and I am confused and frustrated as hell. I've created a program like this before but can't understand what is going on.

I've created a very simple factorial program and have double checked the logic, but every time I enter a number greater than 2 the program goes into a loop constantly printing out "inf". I can't see ANYTHING wrong with the program itself. :(

#include <iostream>

using namespace std;

int main() {
    double userNumber = 0;
    double i = 1;

    cout << "This program will calculate the factorial of the number you enter.\nPlease enter your number now: ";
    cin >> userNumber;

    for ( i = 1; i < userNumber; i++ ) {
        userNumber *= i;
        cout << userNumber;
}

    cout << "\n\nThe factorial is " << userNumber << "." << endl;

    return 0;
}

It works for 1 and 2:

But as soon as you do 3 or greater....

I haven't created a C++ program in a while but can't for the life of me see what is wrong. Is this just a super obvious syntax error, or is my computer finally breaking down on me?

EDIT: I just changed the numbers from double to int and this is what I got:

I still don't understand why it is doing this. I don't see a problem with the for loop, or anything else...

Argh. Smashes head into desk

Answer 1

Change

for ( i = 1; i < userNumber; i++ ) {

to

for ( i = userNumber - 1; i > 1; --i ) {

Note that your code changes userNumber in the loop body, so that the loop will only terminate if userNumber overflows.