# Factorial in Every Language

Published on 01 November 2018 (Updated: 02 November 2018)

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`

.

## Requirements

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.

## Testing

Some tests for your program are:

Description | Input | Output |
---|---|---|

Empty input | “Usage: please input a non-negative integer” | |

Not a number | “asdf” | “Usage: please input a non-negative integer” |

Negative integer | -1 | “Usage: please input a non-negative integer” |

Zero | 0 | 1 |

Positive integer | 1 | 1 |

Positive integer | 4 | 24 |

Positive integer | 8 | 40320 |

Positive integer | 10 | 3628800 |

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