Published on 26 October 2018 (Updated: 22 January 2024)

Welcome to the Factorial page! Here, you'll find a description of the project as well as a list of sample programs written in various languages.

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The factorial of an integer n is defined as:

n! = 1 x 2 x 3 x 4 x ... x n

With the special case 0! = 1.


You must write an executable program that accepts an integer n on standard input via the command line, and outputs n! to standard output.

Note that the factorial function grows very quickly. For example, 4! = 24 but 8! = 40320. Therefore, you should impose a limit on the input, so that the largest factorial still fits into your language's largest supported datatype.

Also note that the factorial is not defined for negative integers.


Every project in the Sample Programs repo should be tested. In this section, we specify the set of tests specific to Factorial. In order to keep things simple, we split up the testing as follows:

Factorial Valid Tests

Description Input Output
Sample Input: Zero "0" "1"
Sample Input: One "1" "1"
Sample Input: Four "4" "24"
Sample Input: Eight "8" "40320"
Sample Input: Ten "10" "3628800"

Factorial Invalid Tests

Description Input
No Input  
Empty Input ""
Invalid Input: Not A Number "asdf"
Invalid Input: Negative "-1"

All of these tests should output the following:

Usage: please input a non-negative integer