The factorial of a number is the product of all the integers from 1 to that number. The factorial can only be defined for positive integers.

The factorial of a negative number doesn't exist. And the factorial of 0 is 1.

For example,

The factorial of a positive number `n`, say `5`

, is denoted by `5!`

and is given by:

`5! = 1 * 2 * 3 * 4 * 5 = 120`

So, the Mathematical logic for factorial is:

```
n! = 1 * 2 * 3 * ... * n
n! = 1 if n = 0 or n = 1
```

In this program below, the user is asked to enter a positive integer. Then the factorial of that number is computed and displayed on the screen.

## Example: Find Factorial of a given number

```
#include <iostream>
using namespace std;
int main()
{
int n;
unsigned long long factorial = 1;
cout << "Enter a positive integer: ";
cin >> n;
if (n < 0)
cout << "Error! Factorial of a negative number doesn't exist.";
else {
for(int i = 1; i <=n; ++i) {
factorial *= i;
}
cout << "Factorial of " << n << " = " << factorial;
}
return 0;
}
```

**Output**

Enter a positive integer: 12 Factorial of 12 = 479001600

In this program, we take a positive integer from the user and compute the factorial using `for`

loop. We print an error message if the user enters a negative number.

We declare the type of factorial variable as `unsigned long`

since the factorial of a number may be very large.

When the user enters a positive integer (say `4`

), `for`

loop is executed and computes the factorial. The value of `i` is initially `1`

.

The program runs until the statement `i <= n`

becomes `false`

. This prints `Factorial of 4 = 24` on the screen. Here’s how the program executes when `n = 4`

.

i <= 4 |
fact *= i |
---|---|

1 <= 4 | fact = 1 * 1 = 1 |

2 <= 4 | fact = 1 * 2 = 2 |

3 <= 4 | fact = 2 * 3 = 6 |

4 <= 4 | fact = 6 * 4 = 24 |

5 <= 4 | Loop terminates. |