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

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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

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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.

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Factorial of a Number

#include 
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

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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.