# C : Find Factorial of a Number

Tutorial by:Manisha Dubey      Date: 2016-06-17 03:20:41

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.

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`

## Example: Factorial of a Number

``````#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.

## C

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