C Program to Find the Factorial of a Number

Written: August 22, 2026

Factorial is the classic loop practice problem: multiply 1 through n and watch the running product grow.

This walkthrough keeps the logic small and readable so you can type it, run it, and then change one input at a time to see what happens.

What you will build

You will read a non-negative integer n and print n! using an iterative product.

• Read n
• Initialize fact = 1
• Multiply fact by each k from 1 to n
• Print the result

Working C example

#include <stdio.h>

int main(void) {
    int n, i;
    unsigned long long fact = 1;

    printf("Enter n: ");
    if (scanf("%d", &n) != 1 || n < 0) {
        printf("Enter a non-negative integer\n");
        return 1;
    }

    for (i = 1; i <= n; i++) {
        fact *= (unsigned long long)i;
    }

    printf("%d! = %llu\n", n, fact);
    return 0;
}

How the logic works

Read the program top to bottom: includes and main first, then the statements that change variables, then the print that proves the result.

If your output looks wrong, print the variables before and after the critical lines. That single habit catches most beginner bugs faster than rewriting the whole file.

Use unsigned long long early — plain int overflows quickly (around 13! on many platforms).

Common mistakes

Forgetting a semicolon, using the wrong format specifier in printf/scanf, and mixing up assignment (=) with comparison (==) are the usual culprits.

Compile with warnings enabled (`gcc -Wall`) so the compiler points at risky casts and unused variables before you chase them by hand.

Try this next

Change the sample inputs, add a second test case, and briefly note what stayed the same. Teaching yourself with tiny experiments sticks better than copying a longer program you never run.

When you can explain every line without looking, you are ready for the next exercise in the series.

Keep learning

If this walkthrough on factorial in c helped, open the code again and change one input or assumption. Small experiments beat rereading the same example.

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