# Find the Factorial!

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.

• 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)? – Ami Feb 6 '11 at 22:43
• @SHiNKiROU, even golfscript can manage 125! less than 1/10th of a second and it's and interpreted interpreted language! – gnibbler Feb 8 '11 at 3:21
• @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. – Michael Stern Jan 7 '14 at 3:18
• Completes in under a minute seems a very hardware-dependent requirement. Completes in under a minute on what hardware? – sergiol Aug 24 '17 at 18:05
• @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. – Kevin Brown Aug 24 '17 at 21:20

## Golfscript -- 12 chars

{,1\{)*}/}:f


### Getting started with Golfscript -- Factorial in step by step

Here's something for the people who are trying to learn golfscript. The prerequisite is a basic understanding of golfscript, and the ability to read golfscript documentation.

So we want to try out our new tool golfscript. It's always good to start with something simple, so we're beginning with factorial. Here's an initial attempt, based on a simple imperative pseudocode:

# pseudocode: f(n){c=1;while(n>1){c*=n;n--};return c}
{:n;1:c;{n 1>}{n c*:c;n 1-:n;}while c}:f


Whitespace is very rarely used in golfscript. The easiest trick to get rid of whitespace is to use different variable names. Every token can be used as a variable (see the syntax page). Useful tokens to use as variables are special characters like |, &, ? -- generally anything not used elsewhere in the code. These are always parsed as single character tokens. In contrast, variables like n will require a space to push a number to the stack after. Numbers are essentially preinitialized variables.

As always, there are going to be statements which we can change, without affecting the end result. In golfscript, everything evaluates to true except 0, [], "", and {} (see this). Here, we can change the loop exit condition to simply {n} (we loop an additional time, and terminate when n=0).

As with golfing any language, it helps to know the available functions. Luckily the list is very short for golfscript. We can change 1- to ( to save another character. At present the code looks like this: (we could be using 1 instead of | here if we wanted, which would drop the initialization.)

{:n;1:|;{n}{n|*:|;n(:n;}while|}:f

It is important to use the stack well to get the shortest solutions (practice practice practice). Generally, if values are only used in a small segment of code, it may not be necessary to store them into variables. By removing the running product variable and simply using the stack, we can save quite a lot of characters.

{:n;1{n}{n*n(:n;}while}:f

Here's something else to think about. We're removing the variable n from the stack at the end of the loop body, but then pushing it immediately after. In fact, before the loop begins we also remove it from the stack. We should instead leave it on the stack, and we can keep the loop condition blank.

{1\:n{}{n*n(:n}while}:f

Maybe we can even eliminate the variable completely. To do this, we will need to keep the variable on the stack at all times. This means that we need two copies of the variable on the stack at the end of the condition check so we don't lose it after the check. Which means that we'll have a redundant 0 on the stack after the loop ends, but that is easy to fix.

This leads us to our optimal while loop solution!

{1\{.}{.@*\(}while;}:f

Now we still want to make this shorter. The obvious target should be the word while. Looking at the documentation, there are two viable alternatives -- unfold and do. When you have a choice of different routes to take, try and weigh the benefits of both. Unfold is 'pretty much a while loop', so as an estimate we'll cut down the 5 character while by 4 into /. As for do, we cut while by 3 characters, and get to merge the two blocks, which might save another character or two.

There's actually a big drawback to using a do loop. Since the condition check is done after the body is executed once, the value of 0 will be wrong, so we may need an if statement. I'll tell you now that unfold is shorter (some solutions with do are provided at the end). Go ahead and try it, the code we already have requires minimal changes.

{1\{}{.@*\(}/;}:f

Great! Our solution is now super-short and we're done here, right? Nope. This is 17 characters, and J has 12 characters. Never admit defeat!

### Now you're thinking with... recursion

Using recursion means we must use a branching structure. Unfortunate, but as factorial can be expressed so succinctly recursively, this seems like a viable alternative to iteration.

# pseudocode: f(n){return n==0?n*f(n-1):1}
{:n{n.(f*}1if}:f # taking advantage of the tokeniser


Well that was easy -- had we tried recursion earlier we may not have even looked at using a while loop! Still, we're only at 16 characters.

### Arrays

Arrays are generally created in two ways -- using the [ and ] characters, or with the , function. If executed with an integer at the top of the stack, , returns an array of that length with arr[i]=i.

For iterating over arrays, we have three options:

1. {block}/: push, block, push, block, ...
2. {block}%: [ push, block, push, block, ... ] (this has some nuances, e.g. intermediate values are removed from the stack before each push)
3. {block}*: push, push, block, push, block, ...

The golfscript documentation has an example of using {+}* to sum the contents of an array. This suggests we can use {*}* to get the product of an array.

{,{*}*}:f


Unfortunately, it isn't quite that simple. All the elements are off by one ([0 1 2] instead of [1 2 3]). We can use {)}% to rectify this issue.

{,{)}%{*}*}:f


Well not quite. This doesn't handle zero correctly. We can calculate (n+1)!/(n+1) to rectify this, although this costs far too much.

{).,{)}%{*}*\/}:f


We can also try to handle n=0 in the same bucket as n=1. This is actual extremely short to do, try and work out the shortest you can.

Not so good is sorting, at 7 characters: [1\]$1=. Note that this sorting technique does has useful purposes, such as imposing boundaries on a number (e.g. [0\100]$1=)
Here's the winner, with only 3 characters: .!+

If we want to have the increment and multiplication in the same block, we should iterate over every element in the array. Since we aren't building an array, this means we should be using {)*}/, which brings us to the shortest golfscript implementation of factorial! At 12 characters long, this is tied with J!

{,1\{)*}/}:f

### Bonus solutions

Starting with a straightforward if solution for a do loop:

{.{1\{.@*\(.}do;}{)}if}:f


We can squeeze a couple extra out of this. A little complicated, so you'll have to convince yourself these ones work. Make sure you understand all of these.

{1\.!!{{.@*\(.}do}*+}:f
{.!{1\{.@*\(.}do}or+}:f
{.{1\{.@*\(.}do}1if+}:f


A better alternative is to calculate (n+1)!/(n+1), which eliminates the need for an if structure.

{).1\{.@*\(.}do;\/}:f

But the shortest do solution here takes a few characters to map 0 to 1, and everything else to itself -- so we don't need any branching. This sort of optimization is extremely easy to miss.

{.!+1\{.@*\(.}do;}:f

For anyone interested, a few alternative recursive solutions with the same length as above are provided here:

{.!{.)f*0}or+}:f
{.{.)f*0}1if+}:f
{.{.(f*}{)}if}:f


*note: I haven't actually tested many of the pieces of code in this post, so feel free to inform if there are errors.

• Interesting, the seems to be a bug in the spoiler markdown when you use code in a spoiler... Anyone cares to mention this on Meta? – Ivo Flipse Feb 7 '11 at 11:55
• I find it interesting how golfscript - a golfing language - allows multi-letter variable names and "punishes" you for using 1 letter with necessary whitespace – Cyoce Feb 4 '16 at 15:45

f n=product[1..n]

• I don't know Haskell... But Will this calculate factorial for 0 – The King Feb 7 '11 at 12:11
• @The King: yes it will. [1..0] ==> [] and product [] ==> 1 – J B Feb 7 '11 at 12:12
• I would argue this uses the "built-in library" that the problem prohibits. Still, the other method f 0=1;f n=n*f$n-1 is 17 characters as well. – eternalmatt Jul 26 '11 at 1:20 • @eternalmatt: that part of the restrictions is underspecified to me. Both product and, say, (*) or (-) "can calculate the factorial", and they're all defined through the Prelude. Why would one be cool and not the other? – J B Jul 27 '11 at 9:28 • @YoYoYonnY: I count 17 characters as well, for less (subjective) readability. IMHO it's fine in the comments. – J B Feb 8 '16 at 13:32 # Python - 27 Just simply: f=lambda x:0**x or x*f(x-1)  • Good trick: 0**x. – Alexandru Jul 11 '11 at 20:25 • What about math.factorial? It isn't a built-in, is it? – user63571 Jan 19 '17 at 20:21 • @JackBates that counts as a builtin, as you didn't write the code to compute the factorial. – FlipTack Jan 21 '17 at 17:51 • Can anyone tell me what's the trick behind 0**x? – Pavitra Aug 4 at 5:36 • @Pavitra: 00=1, and it's the first thing that evaluates so it gets returned. For any other n, 0n=0, thus the first operand of or is falsey, such that the second operand gets evaluated. – Mega Man Aug 5 at 18:13 ## APL (4) ×/∘⍳  Works as an anonymous function:  ×/∘⍳ 5 120  If you want to give it a name, 6 characters: f←×/∘⍳  • I don't speak APL, what is going on here? – Michael Stern Jan 6 '14 at 21:11 • @MichaelStern: ⍳ makes an index vector, i.e. ⍳5 is 1 2 3 4 5. × is (obviously) multiply, / is reduce, and ∘ is function composition. So, ×/∘⍳ is a function that takes an argument x and gives the product of the numbers [1..x]. – marinus Jan 6 '14 at 23:09 • Ah, the same approach as in @Yves Klett's Mathematica solution. Very nice. – Michael Stern Jan 7 '14 at 3:14 • @NBZ: That didn't exist yet in 2011 when this question was written, nor in 2012 when I wrote this answer. Trains were only added in Dyalog 14.0 which came out in 2014. – marinus Jan 25 '16 at 18:38 ## J (12) A standard definition in J: f=:*/@:>:@i.  Less than 1sec for 125! Eg:  f 0 1 f 5 120 f 125x 1882677176888926099743767702491600857595403 6487149242588759823150835315633161359886688 2932889495923133646405445930057740630161919 3413805978188834575585470555243263755650071 31770880000000000000000000000000000000  • why not just */>:i. ? – Andbdrew Aug 21 '11 at 11:26 • Because OP asks for a function and the best we can do in J is to define a verb. – Eelvex Aug 23 '11 at 12:16 • There's no reason it can't be an anonymous function right? Like ([:*/1+i.) for 10 points, or even 8 as the parentheses are only needed for calling the function, not for the definition. – jpjacobs Nov 4 '14 at 22:05 • in the last one, f 125x what does the x do? Is it a special kind of number? – Cyoce Jan 21 '16 at 7:11 • @Cyoce, yes, it's extended precision integer. – Eelvex Jan 21 '16 at 19:23 ## Golfscript - 13 chars (SYM) defines the function ! {),()\{*}/}:! # happy robot version \{*}/  alternate 13 char version {),()+{*}*}:!  whole program version is 10 chars ~),()+{*}*  testcases take less than 1/10 second: input: 0!  output 1  input 125!  output 188267717688892609974376770249160085759540364871492425887598231508353156331613598866882932889495923133646405445930057740630161919341380597818883457558547055524326375565007131770880000000000000000000000000000000  • +1 for symbolic golf entry! I wish I could upvote more than once. :-D – Chris Jester-Young Feb 7 '11 at 0:56 • @ChrisJester-Young I'll do it for you. – Cyoce Jan 21 '16 at 7:30 ## Perl 6: 13 chars $f={[*]1..$_}  [*] is same as Haskell product, and 1..$_ is a count-up from 1 to $_, the argument. • It's not allowed to not use a space after [*] anymore ("Two terms in a row" error message). – Konrad Borowski Dec 28 '13 at 19:24 • You don't need to set a variable, a bare code block is an acceptable answer as it implicitly forms a function. Also does this still work for 0? – Phil H Apr 28 '18 at 7:24 # Matlab, 15 f=@(x)prod(1:x)  Test Cases >> f(0) ans = 1 >> f(4) ans = 24 >> tic,f(125),toc ans = 1.8827e+209 Elapsed time is 0.000380 seconds.  # Python, 28 bytes f=lambda x:x/~x+1or x*f(x-1)  (based off Alexandru's solution) # MATL, 2 bytes :p  Explained: : % generate list 1,2,3,...,i, where i is an implicit input p % calculate the product of of all the list entries (works on an empty list too)  Try it online! • ​​​​​​​​​:O​​​​ – Andras Deak Jan 25 '16 at 12:14 • I was going to post exactly this :-) You may want to modify the link to include the code and an example input – Luis Mendo Jan 31 '16 at 4:56 • As you command, my lord. – flawr Jan 31 '16 at 10:43 • @AndrasDeak, No, that would output all numbers from 1 to i... – YoYoYonnY Feb 6 '16 at 16:09 # Ruby - 21 chars f=->n{n>1?n*f[n-1]:1}  ## Test irb(main):009:0> f=->n{n>1?n*f[n-1]:1} => #<Proc:0x25a6d48@(irb):9 (lambda)> irb(main):010:0> f[125] => 18826771768889260997437677024916008575954036487149242588759823150835315633161 35988668829328894959231336464054459300577406301619193413805978188834575585470555 24326375565007131770880000000000000000000000000000000  ## Java, 85 Chars BigInteger f(int n){return n<2?BigInteger.ONE:new BigInteger(""+n).multiply(f(n-1));}  • This misses the imports: import java.math.*; (so, +19 bytes). – Olivier Grégoire Dec 24 '17 at 10:48 • Fair point. ............ – st0le Dec 29 '17 at 19:36 # PostScript, 26 chars /f{1 exch -1 1{mul}for}def  Example: GS> 0 f = 1 GS> 1 f = 1 GS> 8 f = 40320  The function itself takes only 21 characters; the rest is to bind it to a variable. To save a byte, one can also bind it to a digit, like so: GS> 0{1 exch -1 1{mul}for}def GS> 8 0 load exec = 40320  • Ghostscript cannot handle 125!; anything beyond 34! comes out as 1.#INF. (I used stock GNU Ghostscript 9.0.7 compiled for x64 Windows.) – Ross Presser Jan 7 '14 at 6:17 ## JavaScript, 25 function f(n)!n||n*f(n-1)  ## CoffeeScript, 19 f=(n)->!n||n*f(n-1)  Returns true in the case of n=0, but JavaScript will type-coerce that to 1 anyway. • Don't you need a return statement in the JavaScript function? – Justin Morgan Jul 12 '11 at 19:19 • Update: Holy smoke, you don't need a return! But why not? – Justin Morgan Jul 12 '11 at 19:43 • It's JavaScript 1.8 (developer.mozilla.org/en/new_in_javascript_1.8). Full disclosure, it only works on Firefox! – Casey Chu Jul 13 '11 at 3:53 • Nice, I didn't know about leaving out the return statement for JavaScript 1.8. Also, you can guarantee 1 instead of true for the n=0 case with the same length code: function f(n)n?n*f(--n):1 – Briguy37 Aug 3 '11 at 21:50 • ES6, 17: f=n=>!n||n*f(n-1) Take that, CoffeeScript! – Ry- Dec 29 '13 at 3:03 # Ruby - 30 29 characters def f(n)(1..n).inject 1,:*end  Test f(0) -> 1 f(5) -> 120  • You can put the end directly after :* without a newline or semicolon. – sepp2k Feb 6 '11 at 17:50 • There's no need to pass 1 to the #inject call. (1..10).inject :* #=> 3628800 – Dogbert Feb 7 '11 at 10:56 • @Dogbert, what about for f(0)? – Nemo157 Feb 7 '11 at 19:05 • @Nemo157, ah! forgot about that. – Dogbert Feb 8 '11 at 13:38 • Shorter to use 1.9 lambda syntax: f=->n{(1..n).inject 1,:*}. Call it with f[n]. – Michael Kohl Aug 22 '11 at 22:04 ## F#: 26 chars There's no inbuilt product function in F#, but you can make one with a fold let f n=Seq.fold(*)1{1..n}  # C#, 20 or 39 characters depending on your point of view As a traditional instance method (39 characters; tested here): double f(int x){return 2>x?1:x*f(x-1);}  As a lambda expression (20 characters, but see disclaimer; tested here): f=x=>2>x?1:x*f(x-1);  We have to use double because 125! == 1.88 * 10209, which is much higher than ulong.MaxValue. ### Disclaimer about the lambda version's character count: If you recursion in a C# lambda, you obviously have to store the lambda in a named variable so that it can call itself. But unlike (e.g.) JavaScript, a self-referencing lambda must have been declared and initialized on a previous line. You can't call the function in the same statement in which you declare and/or initialize the variable. In other words, this doesn't work: Func<int,double> f=x=>2>x?1:x*f(x-1); //Error: Use of unassigned local variable 'f'  But this does: Func<int,double> f=null; f=x=>2>x?1:x*f(x-1);  There's no good reason for this restriction, since f can't ever be unassigned at the time it runs. The necessity of the Func<int,double> f=null; line is a quirk of C#. Whether that makes it fair to ignore it in the character count is up to the reader. # CoffeeScript, 21 19 characters for real f=(x)->+!x||x*f x-1  Tested here: http://jsfiddle.net/0xjdm971/ # Brachylog, 7 6 bytes By making a range and multiplying it -1 byte tanks to ovs having the idea to use the max() function ;1⌉⟦₁×  ## Explanation ;1 -- If n<1, use n=1 instead (zero case) ⟦₁ -- Construct the range [1,n] × -- return the product of said range  Try it online! # Brachylog, 10 9 bytes recursion ≤1|-₁↰;?×  ## Explanation  --f(n): ≤1 -- if n ≤ 1: return 1 | -- else: -₁↰ -- f(n-1) ;?× -- *n  Try it online! • This works for 6 bytes. Taking input as a singleton is allowed by default. – ovs Aug 22 '18 at 17:58 • @ovs thanks. But using ; instead of , allows for just a regular numerical input. -1byte anyway – Kroppeb Aug 22 '18 at 18:12 # C (39 chars) double f(int n){return n<2?1:n*f(n-1);}  • Nice. But can save some characters: double f(n){return!n?1:n*f(n-1);} - 33 chars. – ugoren Jan 12 '12 at 16:38 • f(125) will overflow – jkabrg Oct 14 '15 at 7:12 # D: 45 Characters T f(T)(T n){return n < 2 ? 1 : n * f(n - 1);}  More legibly: T f(T)(T n) { return n < 2 ? 1 : n * f(n - 1); }  A cooler (though longer version) is the templatized one which does it all at compile time (64 characters): template F(int n){static if(n<2)enum F=1;else enum F=n*F!(n-1);}  More legibly: template F(int n) { static if(n < 2) enum F = 1; else enum F = n * F!(n - 1); }  Eponymous templates are pretty verbose though, so you can't really use them in code golf very well. D's already verbose enough in terms of character count to be rather poor for code golf (though it actually does really well at reducing overall program size for larger programs). It's my favorite language though, so I figure that I might as well try and see how well I can get it to do at code golf, even if the likes of GolfScript are bound to cream it. • take out the whitespace and you can get it down to 36 chars – ratchet freak Jul 3 '11 at 13:36 • @Cyoce Can you explain? – YoYoYonnY Feb 6 '16 at 16:08 • Welcome to the site, @user272735. Note that we don't edit people's solutions in order to make improvements here. Instead we leave comments suggesting those improvements, as ratchet freak did above. – Shaggy Jul 29 at 8:29 ## PowerShell – 36 Naïve: filter f{if($_){$_*(--$_|f}else{1}}


Test:

> 0,5,125|f
1
120
1,88267717688893E+209


## Scala, 39 characters

def f(x:BigInt)=(BigInt(1)to x).product


Most of the characters are ensuring that BigInts are used so the requirement for values up to 125 is met.

• Some shorter options: (x:Int)=>(BigInt(1)to x).product def f(x:Int)=(BigInt(1)to x).product def f(x:BigInt)=(x.to(1,-1)).product def f(x:BigInt)=(-x to-1).product.abs – LRLucena Jun 7 '16 at 14:32

# Javascript, ES6 17

f=n=>n?n*f(n-1):1


ES6:

• Arrow function
• ES6 is younger than this challenge if I'm remembering correctly and therefore not eligible. – lirtosiast Jun 22 '15 at 19:44
• There is smth strange with conditional operator. Why there are two colons? – Qwertiy Jun 25 '15 at 15:43
• @Qwertiy You're right, that was a typo, thanks. – Afonso Matos Jun 25 '15 at 18:21

## PowerShell, 42 bytes

(saved 2 chars using filter instead of function)

filter f($x){if(!$x){1}else{$x*(f($x-1))}}


Output:

PS C:\> f 0
1
PS C:\> f 5
120
PS C:\> f 1
1
PS C:\> f 125
1.88267717688893E+209

• This is way old now, but... Can save 1 more character by reversing the if/else: filter f($x){if($x){$x*(f($x-1))}else{1}}. And it can be reduced further to 36 characters if it's called via pipeline since it's a filter (e.g. 125|f): filter f{if($_){$_*($_-1|f)}else{1}} – Andrew Oct 14 '15 at 18:31 # Racket (scheme) 4035 29 bytes Computes 0! to be 1, and computes 125! in 0 seconds according to timer. Regular recursive approach (define(f n)(if(= n 0)1(* n(f(- n 1)))))  New version to beat common lisp: multiplies all elements of a list (same as that Haskell solution) (λ(n)(apply *(build-list n add1)))  Newer version to beat the other scheme solution and math the other racket solution by using foldl instead of apply and using range instead of buildlist (λ(n)(foldl * n(range 1 n)))  # Mornington Crescent, 1827 1698 chars I felt like learning a new language today, and this is what I landed on... (Why do I do this to myself?) This entry won't be winning any prizes, but it beats all 0 other answers so far using the same language! Take Northern Line to Bank Take Central Line to Holborn Take Piccadilly Line to Heathrow Terminals 1, 2, 3 Take Piccadilly Line to Acton Town Take District Line to Acton Town Take District Line to Parsons Green Take District Line to Bank Take District Line to Parsons Green Take District Line to Acton Town Take District Line to Hammersmith Take Circle Line to Aldgate Take Circle Line to Aldgate Take Metropolitan Line to Chalfont & Latimer Take Metropolitan Line to Aldgate Take Circle Line to Hammersmith Take District Line to Acton Town Take Piccadilly Line to Bounds Green Take Piccadilly Line to Acton Town Take Piccadilly Line to Bounds Green Take Piccadilly Line to Acton Town Take District Line to Acton Town Take District Line to Bank Take Circle Line to Hammersmith Take District Line to Upminster Take District Line to Parsons Green Take District Line to Notting Hill Gate Take Circle Line to Notting Hill Gate Take Circle Line to Bank Take Circle Line to Temple Take Circle Line to Aldgate Take Circle Line to Aldgate Take Metropolitan Line to Chalfont & Latimer Take Metropolitan Line to Chalfont & Latimer Take Metropolitan Line to Aldgate Take Circle Line to Hammersmith Take District Line to Upminster Take District Line to Bank Take District Line to Upney Take District Line to Upminster Take District Line to Hammersmith Take District Line to Upminster Take District Line to Upney Take District Line to Bank Take Circle Line to Embankment Take Circle Line to Embankment Take Northern Line to Angel Take Northern Line to Moorgate Take Metropolitan Line to Chalfont & Latimer Take Metropolitan Line to Moorgate Take Circle Line to Moorgate Take Northern Line to Mornington Crescent  Try it online! Anyone who's journeyed around London will understand that instantly of course, so I'm sure I don't need to give a full explanation. Most of the work at the start is in handling the 0 case. After initialising the product at 1, I can use that to calculate max(input, 1) to get the new input, taking advantage of the fact that 0! = 1! Then the main loop can begin. (EDIT: A whole bunch of trips have been saved by stripping the 1 from "Heathrow Terminals 1, 2, 3" instead of generating it by dividing 7 (Sisters) by itself. I also use a cheaper method to generate the -1 in the next step.) Decrementing is expensive in Mornington Crescent (although less expensive than the Tube itself). To make things more efficient I generate a -1 by taking the NOT of a parsed 0 and store that in Hammersmith for much of the loop. I put some significant work into this, but since this is my first attempt at golfing in Mornington Crescent (in fact my first attempt in any language), I expect I missed a few optimisations here and there. If you're interested in programming in this language yourself (and why wouldn't you be?), Esoteric IDE - with its debug mode and watch window - is a must! ## Befunge - 2x20 = 40 characters 0\:#v_# 1#<$v *\< >:1-:#^_>\:#^_  This is a function in that it is a standalone block of code not utilising the wraparound. You have to place the argument on the top of the stack then enter from the top-left going right, the function will exit from the bottom-right going right with the result on the top of the stack. E.g. to calculate the factorial of 125 555** 0\:#v_# 1#<$v *\< >:1-:#^_$>\:#^_$.@  Testing 0 0 0\:#v_# 1#<\$v *\<
>:1-:#^_$>\:#^_$    .@

• I know this is pretty old, but I think this is somewhat shorter and quicker : &:!#@_>:# 1# -# :# _$>\# :#* _$.@ (where & should be replaced by the input). It's 32 chars/bytes – FliiFe Apr 7 '16 at 11:08

# J - 6 characters

*/>:i.


Does this count? I know it is very similar to the earlier J example, but it is a little shorter :)

I'm a beginner with J, but it's a lot of fun so far!

# In C (23 Characters)

This abuses the GCC "feature" that makes the last assignment count as a return if no return is specified.

f(a){a=a>0?f(a-1)*a:1;}


In proper C, 28 characters

f(a){return a>0?f(a-1)*a:1;}

• +1 for the GCC "feature". I think GCC even allows a block return value (Can remember doing something like this) 0 == ({printf("Hello, world!"); 0;}); – YoYoYonnY Feb 6 '16 at 16:11

## Kona (11 6)

*/1.+!


K works right-to-left (for the most part), so we enumerate x (make a list/array of numbers from 0 to x-1), add 1 to it (list ranges 0 to x), then multiply all numbers together. If it weren't a requirement to compute 125!, I could save 1 more byte by eliminating . next to the 1. In any event, 125! is computed in mere milliseconds:

  */1.+!125.
1.882677e+209

• You don't need a lot of this. K has currying, so the entire answer becomes */1.+!`: 6 bytes. – kirbyfan64sos Jun 25 '15 at 14:43
• @kirbyfan64sos: True & I'll edit it in. I think when I wrote this ~18 months ago, I was still stuck on everything must be callable (i.e., function). – Kyle Kanos Jun 25 '15 at 15:00