RCU 21BCA1C1P · Lab Program 4

Factorial of a Number in C

Read a non-negative integer and print its factorial, the product of all whole numbers from 1 up to it.

Algorithm

Read n and reject negative values.
Set result to 1.
Multiply result by every integer from 2 up to n.
Print result.

C program

#include <stdio.h>

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

    printf("Enter a non-negative integer (0-20):\n");
    if (scanf("%d", &n) != 1 || n < 0 || n > 20) {
        printf("Error: enter a value from 0 to 20\n");
        return 1;
    }

    for (i = 2; i <= n; i++) {
        result *= i;
    }

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

Sample input and output

Factorial of 5

Input

Output

Enter a non-negative integer (0-20):
5! = 120

Edge case: 0! is 1

Input

Output

Enter a non-negative integer (0-20):
0! = 1

How the program works

The loop starts at 2 because multiplying by 1 changes nothing and result already starts at 1. For n = 0 the loop does not run, which gives the defined value 0! = 1.

Factorials grow very fast. 20! is the largest that fits in an unsigned 64-bit integer, which is why the input is limited to 20.

Run this code

Edit the program and press Run. It is free and needs no sign-in.

Program input (what the program reads with scanf)

Copy a sample and paste it into the STDIN box in the editor before you press Run.

Factorial of 5

Expected output

Enter a non-negative integer (0-20):
5! = 120

Edge case: 0! is 1

Expected output

Enter a non-negative integer (0-20):
0! = 1

Viva questions

What is 0! and why?

1, by convention; it makes the recurrence n! = n × (n - 1)! work for n = 1.

Why is unsigned long long used for the result?

It holds much larger values than int, and 20! is the largest factorial that fits.

Why is the loop bounded at 20?

21! overflows a 64-bit integer, which would give a wrong answer.

Is recursion an alternative?

Yes: factorial(n) = n * factorial(n - 1) with factorial(0) = 1.

What does %llu mean in printf?

It prints an unsigned long long value.

Common mistakes

Using int for the result, which overflows from 13! onwards.
Starting the result at 0, which makes every factorial 0.

Back to all RCU 21BCA1C1P lab programs.

C program

#include <stdio.h> int main(void) { int n, i; unsigned long long result = 1; printf("Enter a non-negative integer (0-20):\n"); if (scanf("%d", &n) != 1 || n < 0 || n > 20) { printf("Error: enter a value from 0 to 20\n"); return 1; } for (i = 2; i <= n; i++) { result *= i; } printf("%d! = %llu\n", n, result); return 0; }

Viva questions

What is 0! and why?

1, by convention; it makes the recurrence n! = n × (n - 1)! work for n = 1.

Why is unsigned long long used for the result?

It holds much larger values than int, and 20! is the largest factorial that fits.

Why is the loop bounded at 20?

21! overflows a 64-bit integer, which would give a wrong answer.

Is recursion an alternative?

Yes: factorial(n) = n * factorial(n - 1) with factorial(0) = 1.

What does %llu mean in printf?

It prints an unsigned long long value.