Help with my Factorial Program

[ReDoX]

I'm very new to C++ and I'm making a program that calculates the factorial of a number. Just in case someone doesn't know, the factorial of a number is the sum of all numbers starting at 1 and ending at the number itself. Eg. The Factorial of 3 = 1 + 2 + 3 = 6.
So here is my code uptill now:

#include <iostream>
using namespace std;
void Welcome(double& num)
{
     cout << "Hello! This program will help you calculate the factorial of any number" << endl << "Enter any positive number: ";
     cin >> num;
     return;
}
void Check(double& num)
{
     for (int i = 0; i < 1;)
     {
         int a = int(num);
         if (num - a != 0 || num < 0)
         {
            if (num - a != 0)
            {
                cout << "Error. Enter an integer: ";
                cin >> num;
                a = int(num);
            }
            if (num < 0)
            {
                cout << "Error. Enter a positive number: ";
                cin >> num;
            }
            if (num >= 0 && num - a == 0) break;              
         }
     }
     return;
}
main()
{
      double num;
      Welcome(num);
      Check(num);
      cout << num << endl; // Only checking if the above functions are working
      system("Pause");
      return 0;
}

When I compile that, the Welcome function works fine. But when I enter 5 (For example), a valid input, the cursor goes to the next line and keeps blinking:

Hello! This program will help you calculate the factorial of any number
Enter any positive number: 5

If I enter -5, it will ask me to enter a positive number. Then if I enter 4, for example, it hangs again:

Hello! This program will help you calculate the factorial of any number
Enter any positive number: -5
Error. Enter a positive number: 4

But lets say I enter 4.5. It will ask me to enter an integer, so I enter 5. Then the program works fine and outputs 5:

Hello! This program will help you calculate the factorial of any number
Enter any positive number: 4.5
Error. Enter an integer: 5
5
Press any key to continue . . .

What's wrong?

[iambehnam]

Hello.
You don't have calculate 1+2+...+num . why?
don't you use for() instead of three if() in your check function?
use this code too:

int k;
while(num>0){
k+=num;
num--;
}

good luck;

[ReDoX]

I already made a simple program like that once but now I wanted to make it more complex. I want to the user to be able to enter any number he wants and the program should decide whether or not the input is valid. As I said before, the above code is not complete. The calculations aren't done yet. And maybe unknowingly, you answered my question! I had an extra 'if' for some reason.

if (num - a != 0 || num < 0) // This one was unnecessary
         {
            if (num - a != 0)
            {
                cout << "Error. Enter an integer: ";
                cin >> num;
                a = int(num);
            }
            if (num < 0)
            {
                cout << "Error. Enter a positive number: ";
                cin >> num;
            }
            if (num >= 0 && num - a == 0) break;              
         }

HAHA. God knows why I did that. I resumed this program after a long time, maybe that's why. But Thank you!

[Chervil]

Eg. The Factorial of 3 = 1 + 2 + 3 = 6.

Actually that's the sum of the integers. The factorial is the product of the integers.
The Factorial of 3 = 1 * 2 * 3 = 6.

It happens in this case to give the same result, 6.

But 4! = 1*2*3*4 = 24

[nathan10]

Actually that's the sum of the integers. The factorial is the product of the integers.
The Factorial of 3 = 1 * 2 * 3 = 6.

It happens in this case to give the same result, 6.

But 4! = 1*2*3*4 = 24

Furthermore, no huge program is needed to compute the sum of positive integers between any two values. Carl Gauss developed a neat formula for that.

sum = n * (n+1) / 2; when the lower integer is 1.

sum = (upper integer - lower integer + 1) * (lower integer + upper integer) / 2; when the lower integer is greater than 1.