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C Program to Find Factorial of a Number

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:


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 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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