In this
example, you will learn to calculate the factorial of a number entered by the
user.

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

C Data Types

C Programming Operators

C if...else Statement

C for Loop

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

**Factorial of a Number**

```
#include <stdio.h>
```
int main() {
int n, i;
unsigned long long fact = 1;
printf("Enter an integer: ");
scanf("%d", &n);
// shows 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) {
fact *= i;
}
printf("Factorial of %d = %llu", n, fact);
}
return 0;
}

**Output**

Enter an integer: 10

Factorial of 10 = 3628800

This program
takes a positive integer from the user and computes the 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 a
negative number, the program displays a custom error message.

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

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