Example: Factorial using Recursion in C

C
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// Factorial using Recursion
#include <stdio.h>

// Function to calculate factorial using recursion
long long factorial(int n) {
    // Step 1: Define the base case
    // Factorial of 0 is 1
    if (n == 0) {
        return 1;
    }
    // Step 2: Define the recursive step
    // n! = n * (n-1)!
    else {
        return (long long)n * factorial(n - 1);
    }
}

int main() {
    int num;
    long long result;

    // Step 1: Prompt the user for input
    printf("Enter a non-negative integer: ");
    scanf("%d", &num);

    // Step 2: Validate input
    if (num < 0) {
        printf("Factorial is not defined for negative numbers.\n");
    } else {
        // Step 3: Call the recursive factorial function
        result = factorial(num);
        // Step 4: Display the result
        printf("Factorial of %d = %lld\n", num, result);
    }

    return 0;
}