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Create the shortest program or function that finds the factorial of a non-negative integer.

The factorial, represented with ! is defined as such

$$n!:=\begin{cases}1 & n=0\\n\cdot(n-1)!&n>0\end{cases}$$

In plain English the factorial of 0 is 1 and the factorial of n, where n is larger than 0 is n times the factorial of one less than n.

Your code should perform input and output using a standard methods.

Requirements:

  • Does not use any built-in libraries that can calculate the factorial (this includes any form of eval)
  • Can calculate factorials for numbers up to 125
  • Can calculate the factorial for the number 0 (equal to 1)
  • Completes in under a minute for numbers up to 125

The shortest submission wins, in the case of a tie the answer with the most votes at the time wins.

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  • 11
    \$\begingroup\$ How many of the given answers can actually compute up to 125! without integer overflow? Wasn't that one of the requirements? Are results as exponential approximations acceptable (ie 125 ! = 1.88267718 × 10^209)? \$\endgroup\$ – Ami Feb 6 '11 at 22:43
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    \$\begingroup\$ @SHiNKiROU, even golfscript can manage 125! less than 1/10th of a second and it's and interpreted interpreted language! \$\endgroup\$ – gnibbler Feb 8 '11 at 3:21
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    \$\begingroup\$ @ugoren the two-character solution to the other question uses a built-in factorial function. That's not allowed in this version of the challenge. \$\endgroup\$ – Michael Stern Jan 7 '14 at 3:18
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    \$\begingroup\$ Completes in under a minute seems a very hardware-dependent requirement. Completes in under a minute on what hardware? \$\endgroup\$ – sergiol Aug 24 '17 at 18:05
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    \$\begingroup\$ @sergiol Incredibly that hasn't been an issue in the last 2 years, I suspect most languages can get it done in under a minute. \$\endgroup\$ – Kevin Brown Aug 24 '17 at 21:20

181 Answers 181

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W, 5 bytes

*R:e+

Explanation

*R    % Reduce by the product
  :e  % Is this item 0?
    + % Add this value to the product
      % We need to also calculate fact(0)
      % Implicit output
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