The factorial of a positive integer n is equal to 1*2*3*...n. You will learn to calculate the factorial of a number using for loop in this example.

To understand this example, you should have the knowledge of following C programming topics:

The factorial of a positive number n is given by:

factorial of n (n!) = 1*2*3*4....n

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

```
#include <stdio.h>
int main()
{
int n, i;
unsigned long long factorial = 1;
printf("Enter an integer: ");
scanf("%d",&n);
// show error if the user enters a negative integer
if (n < 0)
printf("Error! Factorial of a negative number doesn't exist.");
else
{
for(i=1; i<=n; ++i)
{
factorial *= i; // factorial = factorial*i;
}
printf("Factorial of %d = %llu", n, factorial);
}
return 0;
}
```

**Output**

Enter an integer: 10 Factorial of 10 = 3628800

This program takes a positive integer from the user and computes factorial using for loop.

Since the factorial of a number may be very large, the type of `factorial` variable is declared as `unsigned long long`

.

If the user enters negative number, the program displays error message.

You can also find the factorial of a number using recursion.