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The Factorial Function of a positive integer, n, is defined as the product of the sequence n, n-1, n-2, ...1 and the factorial of zero, 0, is defined as being 1.

Write a function to return the factorial of a number. Solutions can be iterative or recursive (though recursive solutions are generally considered too slow and are mostly used as an exercise in recursion). Support for trapping negative n errors is optional.

ABAP

Iterative

<lang ABAP>form factorial using iv_val type i.

 data: lv_res type i value 1.
 do iv_val times.
   multiply lv_res by sy-index.
 enddo.

 iv_val = lv_res.

endform.</lang>

Recursive

<lang ABAP>form fac_rec using iv_val type i.

 data: lv_temp type i.

 if iv_val = 0.
   iv_val = 1.
 else.
   lv_temp = iv_val - 1.
   perform fac_rec using lv_temp.
   multiply iv_val by lv_temp.
 endif.

endform.</lang>

ActionScript

Iterative

<lang actionscript>public static function factorial(n:int):int {

   if (n < 0)
       return 0;

   var fact:int = 1;
   for (var i:int = 1; i <= n; i++)
       fact *= i;

   return fact;

}</lang>

Recursive

<lang actionscript>public static function factorial(n:int):int {

  if (n < 0)
      return 0;

  if (n == 0)
      return 1;
  
  return n * factorial(n - 1);

}</lang>

Ada

Iterative

<lang ada>function Factorial (N : Positive) return Positive is

  Result : Positive := N;
  Counter : Natural := N - 1;

begin

  for I in reverse 1..Counter loop
     Result := Result * I;
  end loop;
  return Result;

end Factorial;</lang>

Recursive

<lang ada>function Factorial(N : Positive) return Positive is

  Result : Positive := 1;

begin

  if N > 1 then
     Result := N * Factorial(N - 1);
  end if;
  return Result;

end Factorial;</lang>

Numerical Approximation

<lang ada>with Ada.Numerics.Generic_Complex_Types; with Ada.Numerics.Generic_Complex_Elementary_Functions; with Ada.Numerics.Generic_Elementary_Functions; with Ada.Text_IO.Complex_Io; with Ada.Text_Io; use Ada.Text_Io;

procedure Factorial_Numeric_Approximation is

  type Real is digits 15;
  package Complex_Pck is new Ada.Numerics.Generic_Complex_Types(Real);
  use Complex_Pck;
  package Complex_Io is new Ada.Text_Io.Complex_Io(Complex_Pck);
  use Complex_IO;
  package Cmplx_Elem_Funcs is new Ada.Numerics.Generic_Complex_Elementary_Functions(Complex_Pck);
  use Cmplx_Elem_Funcs;
  
  function Gamma(X : Complex) return Complex is
     package Elem_Funcs is new Ada.Numerics.Generic_Elementary_Functions(Real);
     use Elem_Funcs;
     use Ada.Numerics;
     -- Coefficients used by the GNU Scientific Library
     G : Natural := 7;
     P : constant array (Natural range 0..G + 1) of Real := (
        0.99999999999980993, 676.5203681218851, -1259.1392167224028,
        771.32342877765313, -176.61502916214059, 12.507343278686905,
        -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7);
     Z : Complex := X;
     Cx : Complex;
     Ct : Complex;
  begin
     if Re(Z) < 0.5 then
        return Pi / (Sin(Pi * Z) * Gamma(1.0 - Z));
     else
        Z := Z - 1.0;
        Set_Re(Cx, P(0));
        Set_Im(Cx, 0.0);
        for I in 1..P'Last loop
           Cx := Cx + (P(I) / (Z + Real(I)));
        end loop;
        Ct := Z + Real(G) + 0.5;
        return Sqrt(2.0 * Pi) * Ct**(Z + 0.5) * Exp(-Ct) * Cx;
     end if;
  end Gamma;
  
  function Factorial(N : Complex) return Complex is
  begin
     return Gamma(N + 1.0);
  end Factorial;
  Arg : Complex;

begin

  Put("factorial(-0.5)**2.0 = ");
  Set_Re(Arg, -0.5);
  Set_Im(Arg, 0.0);
  Put(Item => Factorial(Arg) **2.0, Fore => 1, Aft => 8, Exp => 0);
  New_Line;
  for I in 0..9 loop
     Set_Re(Arg, Real(I));
     Set_Im(Arg, 0.0);
     Put("factorial(" & Integer'Image(I) & ") = ");
     Put(Item => Factorial(Arg), Fore => 6, Aft => 8, Exp => 0);
     New_Line;
  end loop;

end Factorial_Numeric_Approximation;</lang> Output:

factorial(-0.5)**2.0 = (3.14159265,0.00000000)
factorial( 0) = (     1.00000000,     0.00000000)
factorial( 1) = (     1.00000000,     0.00000000)
factorial( 2) = (     2.00000000,     0.00000000)
factorial( 3) = (     6.00000000,     0.00000000)
factorial( 4) = (    24.00000000,     0.00000000)
factorial( 5) = (   120.00000000,     0.00000000)
factorial( 6) = (   720.00000000,     0.00000000)
factorial( 7) = (  5040.00000000,     0.00000000)
factorial( 8) = ( 40320.00000000,     0.00000000)
factorial( 9) = (362880.00000000,     0.00000000)

Aime

Iterative

<lang aime>integer factorial(integer n) {

   integer i, result;

   result = 1;
   i = 1;
   while (i < n) {
       i += 1;
       result *= i;
   }

   return result;

}</lang>

ALGOL 68

Iterative

<lang algol68>PROC factorial = (INT upb n)LONG LONG INT:(

 LONG LONG INT z := 1;
 FOR n TO upb n DO z *:= n OD;
 z

); ~</lang>

Numerical Approximation

Works with: ALGOL 68 version Revision 1 - no extensions to language used

Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny

<lang algol68>INT g = 7; []REAL p = []REAL(0.99999999999980993, 676.5203681218851, -1259.1392167224028,

               771.32342877765313,   -176.61502916214059,     12.507343278686905, 
                -0.13857109526572012,   9.9843695780195716e-6, 1.5056327351493116e-7)[@0];

PROC complex gamma = (COMPL in z)COMPL: (

 # Reflection formula #
 COMPL z := in z;
 IF re OF z < 0.5 THEN
   pi / (complex sin(pi*z)*complex gamma(1-z))
 ELSE
   z -:= 1;
   COMPL x := p[0];
   FOR i TO g+1 DO x +:= p[i]/(z+i) OD;
   COMPL t := z + g + 0.5;
   complex sqrt(2*pi) * t**(z+0.5) * complex exp(-t) * x
 FI

);

OP ** = (COMPL z, p)COMPL: ( z=0|0|complex exp(complex ln(z)*p) ); PROC factorial = (COMPL n)COMPL: complex gamma(n+1);

FORMAT compl fmt = $g(-16, 8)"⊥"g(-10, 8)$;

test:(

 printf(($q"factorial(-0.5)**2="f(compl fmt)l$, factorial(-0.5)**2));
 FOR i TO 9 DO
   printf(($q"factorial("d")="f(compl fmt)l$, i, factorial(i)))
 OD

) </lang> Output:

 factorial(-0.5)**2=      3.14159265⊥0.00000000
 factorial(1)=      1.00000000⊥0.00000000
 factorial(2)=      2.00000000⊥0.00000000
 factorial(3)=      6.00000000⊥0.00000000
 factorial(4)=     24.00000000⊥0.00000000
 factorial(5)=    120.00000000⊥0.00000000
 factorial(6)=    720.00000000⊥0.00000000
 factorial(7)=   5040.00000000⊥0.00000000
 factorial(8)=  40320.00000000⊥0.00000000
 factorial(9)= 362880.00000000⊥0.00000000

Recursive

<lang algol68>PROC factorial = (INT n)LONG LONG INT:

 CASE n+1 IN
   1,1,2,6,24,120,720 # a brief lookup #
 OUT
   n*factorial(n-1)
 ESAC

~</lang>

AmigaE

Recursive solution: <lang amigae>PROC fact(x) IS IF x>=2 THEN x*fact(x-1) ELSE 1

PROC main()

 WriteF('5! = \d\n', fact(5))

ENDPROC</lang>

Iterative: <lang amigae>PROC fact(x)

 DEF r, y
 IF x < 2 THEN RETURN 1
 r := 1; y := x;
 FOR x := 2 TO y DO r := r * x

ENDPROC r</lang>

AppleScript

Iterative

<lang AppleScript>on factorial(x) if x < 0 then return 0 set R to 1 repeat while x > 1 set {R, x} to {R * x, x - 1} end repeat return R end factorial</lang>

Recursive

<lang AppleScript>on factorial(x) if x < 0 then return 0 if x > 1 then return x * (my factorial(x - 1)) return 1 end factorial</lang>

AutoHotkey

Iterative

<lang AutoHotkey>MsgBox % factorial(4)

factorial(n) {

 result := 1 
 Loop, % n
   result *= A_Index
 Return result

}</lang>

Recursive

<lang AutoHotkey>MsgBox % factorial(4)

factorial(n) {

 return n > 1 ? n-- * factorial(n) : 1

}</lang>

AutoIt

Iterative

<lang AutoIt>;AutoIt Version: 3.2.10.0 MsgBox (0,"Factorial",factorial(6)) Func factorial($int)

   If $int < 0 Then
     Return 0
  EndIf
  $fact = 1
  For $i = 1 To $int
       $fact = $fact * $i
  Next
  Return $fact

EndFunc </lang>

Recursive

<lang AutoIt>;AutoIt Version: 3.2.10.0 MsgBox (0,"Factorial",factorial(6)) Func factorial($int)

  if $int < 0 Then
     return 0
  Elseif $int == 0 Then
     return 1
  EndIf
  return $int * factorial($int - 1)

EndFunc </lang>

AWK

Recursive <lang awk>function fact_r(n) {

 if ( n <= 1 ) return 1;
 return n*fact_r(n-1);

}</lang>

Iterative <lang awk>function fact(n) {

 if ( n < 1 ) return 1;
 r = 1
 for(m = 2; m <= n; m++) {
   r *= m;
 }
 return r

}</lang>

BASIC

Iterative

Works with: QBasic

Works with: RapidQ

<lang freebasic> FUNCTION factorial (n AS Integer) AS Integer

   DIM f AS Integer, i AS Integer
   f = 1
   FOR  i = 2 TO n
       f = f*i
   NEXT i
   factorial = f

END FUNCTION </lang>

Recursive

Works with: QBasic

Works with: RapidQ

<lang freebasic> FUNCTION factorial (n AS Integer) AS Integer

   IF n < 2 THEN
       factorial = 1
   ELSE
       factorial = n * factorial(n-1)
   END IF

END FUNCTION </lang>

Batch File

   @echo off
   set /p x=
   set /a fs=%x%-1
   set y=%x%
   FOR /L %%a IN (%fs%, -1, 1) DO SET /a y*=%%a
   if %x% EQU 0 set y=1
   echo %y%
   pause
   exit

bc

<lang bc>#! /usr/bin/bc -q

define f(x) {

 if (x <= 1) return (1); return (f(x-1) * x)

} f(1000) quit</lang>

Befunge

<lang befunge>&1\> :v v *<

  ^-1:_$>\:|
        @.$<</lang>

Brainf***

Brat

<lang brat>factorial = { x |

 true? x == 0 1 { x * factorial(x - 1)}

}</lang>

C

Iterative

<lang c>int factorial(int n) {

   int result = 1;
   for (int i = 1; i <= n; ++i)
       result *= i;
   return result;

}</lang>

Recursive

<lang c>int factorial(int n) {

   return n == 0 ? 1 : n * factorial(n - 1);

}</lang>

Tail Recursive

Safe with some compilers (for example: GCC with -O2, LLVM's clang)

<lang c>int fac_aux(int n, int acc) {

   return n < 1 ? acc : fac_aux(n - 1, acc * n);

}

int factorial(int n) {

   return fac_aux(n, 1);

}</lang>

C++

The C versions work unchanged with C++, however, here is another possibility using the STL and boost: <lang cpp>#include <boost/iterator/counting_iterator.hpp>

include <algorithm>

int factorial(int n) {

 // last is one-past-end
 return std::accumulate(boost::counting_iterator<int>(1), boost::counting_iterator<int>(n+1), 1, std::multiplies<int>());

}</lang>

Iterative

This version of the program is iterative, with a do-while loop. <lang cpp> long long int Factorial(long long int m_nValue)

  {
      long long int result=m_nValue;
      long long int result_next;
      long long int pc = m_nValue;
      do
      {
          result_next = result*(pc-1);
          result = result_next;
          pc--;
      }while(pc>2);
      m_nValue = result;
      return m_nValue;
  }

</lang>

Template

<lang cpp> template <int N> struct Factorial {

   enum { value = N * Factorial<N - 1>::value };

};

template <> struct Factorial<0> {

   enum { value = 1 };

};

// Factorial<4>::value == 24 // Factorial<0>::value == 1 void foo() {

   int x = Factorial<4>::value; // == 24
   int y = Factorial<0>::value; // == 1

} </lang>

C#

Iterative

<lang csharp>using System;

class Program {

   static int Factorial(int number)
   {
       int accumulator = 1;
       for (int factor = 1; factor <= number; factor++)
       {
           accumulator *= factor;
       }
       return accumulator;
   }

   static void Main()
   {
       Console.WriteLine(Factorial(10));
   }

}</lang>

Recursive

<lang csharp>using System;

class Program {

   static int Factorial(int number)
   {
       return number == 0 ? 1 : number * Factorial(number - 1);
   }

   static void Main()
   {
       Console.WriteLine(Factorial(10));
   }

}</lang>

Tail Recursive

<lang csharp>using System;

class Program {

   static int Factorial(int number)
   {
       return Factorial(number, 1);
   }

   static int Factorial(int number, int accumulator)
   {
       return number == 0 ? accumulator : Factorial(number - 1, number * accumulator);
   }

   static void Main()
   {
       Console.WriteLine(Factorial(10));
   }

}</lang>

Functional

<lang csharp>using System; using System.Linq;

class Program {

   static int Factorial(int number)
   {
       return Enumerable.Range(1, number).Aggregate((accumulator, factor) => accumulator * factor);
   }

   static void Main()
   {
       Console.WriteLine(Factorial(10));
   }

}</lang>

Clay

Obviously there’s more than one way to skin a cat. Here’s a selection — recursive, iterative, and “functional” solutions. <lang Clay> factorialRec(n) {

   if (n == 0) return 1;
   return n * factorialRec(n - 1);

}

factorialIter(n) {

   for (i in range(1, n))
       n *= i;
   return n;

}

factorialFold(n) {

   return reduce(multiply, 1, range(1, n + 1));

} </lang>

We could also do it at compile time, because — hey — why not?

<lang Clay> [n|n > 0] factorialStatic(static n) = n * factorialStatic(static n - 1); overload factorialStatic(static 0) = 1; </lang>

Because a literal 1 has type Int32, these functions receive and return numbers of that type. We must be a bit more careful if we wish to permit other numeric types (e.g. for larger integers).

<lang Clay> [N|Integer?(N)] factorial(n: N) {

   if (n == 0) return N(1);
   return n * factorial(n - 1);

} </lang>

And testing:

<lang Clay> main() {

   println(factorialRec(5));           // 120
   println(factorialIter(5));          // 120
   println(factorialFold(5));          // 120
   println(factorialStatic(static 5)); // 120
   println(factorial(Int64(20)));      // 2432902008176640000

} </lang>

Chef

<lang Chef>Caramel Factorials.

Only reads one value.

Ingredients. 1 g Caramel 2 g Factorials

Method. Take Factorials from refrigerator. Put Caramel into 1st mixing bowl. Verb the Factorials. Combine Factorials into 1st mixing bowl. Verb Factorials until verbed. Pour contents of the 1st mixing bowl into the 1st baking dish.

Serves 1.</lang>

CLIPS

(deffunction factorial (?a)
   (if (or (not (integerp ?a)) (< ?a 0)) then
       (printout t "Factorial Error!" crlf)
    else
       (if (= ?a 0) then
           1
        else
           (* ?a (factorial (- ?a 1))))))

Clojure

Folding

<lang lisp>(defn factorial [x]

 (apply * (range 2 (inc x))))</lang>

Recursive

<lang lisp>(defn factorial [x]

 (if (< x 2)
     1
     (* x (factorial (dec x)))))</lang>

Tail recursive

<lang lisp>(defn factorial [x]

 (loop [x x
        acc 1]
   (if (< x 2)
       acc
       (recur (dec x) (* acc x)))))</lang>

Common Lisp

Recursive: <lang lisp>(defun fact (n)

 (if (< n 2)
     1
   (* n (fact(- n 1)))))

</lang>

Iterative: <lang lisp>(defun factorial (n)

 "Calculates N!"
 (loop for result = 1 then (* result i)
    for i from 2 to n 
    finally (return result)))</lang>

Functional: <lang lisp>(defun factorial (n)

   (reduce #'* (loop for i from 1 to n collect i)))</lang>

CoffeeScript

Several solutions are possible in JavaScript:

Recursive

 if n <= 1
   1
 else
   n * fac n-1</lang>

Functional

Works with: JavaScript version 1.8

(See MDC)

 [1..n].reduce (x,y) -> x*y</lang>

D

<lang D>import std.stdio: writeln; import std.metastrings: toStringNow; import std.algorithm: reduce, iota;

// iterative int factorial(int n) {

   int result = 1;
   foreach (i; 1 .. n+1)
       result *= i;
   return result;

}

// recursive int recFactorial(int n) {

   if (n == 0)
       return 1;
   else
       return n * recFactorial(n - 1);

}

// functional-style int fact(int n) {

   return reduce!q{a * b}(iota(1, n+1));

}

// tail recursive (at run-time, with DMD) int tfactorial(int n) {

   static int facAux(int n, int acc) {
       if (n < 1)
           return acc;
       else
           return facAux(n - 1, acc * n);
   }
   return facAux(n, 1);

}

// computed and printed at compile-time pragma(msg, toStringNow!(factorial(15))); pragma(msg, toStringNow!(recFactorial(15))); pragma(msg, toStringNow!(fact(15))); pragma(msg, toStringNow!(tfactorial(15)));

void main() {

   // computed and printed at run-time
   writeln(factorial(15));
   writeln(recFactorial(15));
   writeln(fact(15));
   writeln(tfactorial(15));

}</lang>

Delphi

Iterative

<lang Delphi>program Factorial1;

{$APPTYPE CONSOLE}

uses SysUtils;

function FactorialIterative(aNumber: Integer): Int64; var

 i: Integer;

begin

 Result := 1;
 for i := 1 to aNumber do
   Result := i * Result;

end;

begin

 Writeln('5! = ' + IntToStr(FactorialIterative(5)));

end.</lang>

Recursive

<lang Delphi>program Factorial2;

{$APPTYPE CONSOLE}

uses SysUtils;

function FactorialRecursive(aNumber: Integer): Int64; begin

 if aNumber < 1 then
   Result := 1
 else
   Result := aNumber * FactorialRecursive(aNumber - 1);

end;

begin

 Writeln('5! = ' + IntToStr(FactorialRecursive(5)));

end.</lang>

Tail Recursive

<lang Delphi>program Factorial3;

{$APPTYPE CONSOLE}

uses SysUtils;

function FactorialTailRecursive(aNumber: Integer): Int64;

　 function FactorialHelper(aNumber: Integer; aAccumulator: Int64): Int64; 　　begin

   if aNumber = 0 then
 　　  Result := aAccumulator
   else
     Result := FactorialHelper(aNumber - 1, aNumber * aAccumulator);
 　　end;

begin

 if aNumber < 1 then
   Result := 1
 else
   Result := FactorialHelper(aNumber, 1);

end;

begin

 Writeln('5! = ' + IntToStr(FactorialTailRecursive(5)));

end.</lang>

DWScript

Note that Factorial is part of the standard DWScript maths functions.

Iterative

<lang delphi>function IterativeFactorial(n : Integer) : Integer; var

  i : Integer;

begin

  Result := 1;
  for i := 2 to n do
     Result *= i;

end; </lang>

Recursive

<lang delphi>function RecursiveFactorial(n : Integer) : Integer; begin

  if n>1 then
     Result := RecursiveFactorial(n-1)*n
  else Result := 1;

end;</lang>

Dylan

<lang dylan>define method factorial(n)

 reduce1(\*, range(from: 1, to: n));

end</lang>

E

<lang e>pragma.enable("accumulator") def factorial(n) {

 return accum 1 for i in 2..n { _ * i }

}</lang>

Ela

Tail recursive version:

<lang Ela>let fact = fact' 1L

          where fact' acc 0 = acc                  
                fact' acc n = fact' (n * acc) (n - 1)</lang>

Emacs Lisp

<lang lisp> (defun fact (n)

 "n is an integer, this function returns n!, that is n * (n - 1)

(n - 2)....* 4 * 3 * 2 * 1"

 (cond
  ((= n 1) 1)
  (t (* n (fact (1- n))))))

</lang>

Erlang

With a fold: <lang erlang>lists:foldl(fun(X,Y) -> X*Y end, 1, lists:seq(1,N)).</lang>

With a recursive function: <lang erlang>fac(1) -> 1; fac(N) -> N * fac(N-1).</lang>

With a tail-recursive function: <lang erlang>fac(N) -> fac(N-1,N). fac(1,N) -> N; fac(I,N) -> fac(I-1,N*I).</lang>

Euphoria

Straight forward methods

Iterative

<lang Euphoria> function factorial(integer n)

 atom f = 1
 while n > 1 do
   f *= n
   n -= 1
 end while

 return f

end function </lang>

Recursive

<lang Euphoria> function factorial(integer n)

 if n > 1 then
   return factorial(n-1) * n
 else
   return 1
 end if

end function </lang>

Tail Recursive

Works with: Euphoria 4.0.0

<lang Euphoria> function factorial(integer n, integer acc = 1)

 if n <= 0 then
   return acc
 else
   return factorial(n-1, n*acc)
 end if

end function </lang>

'Paper tape' / Virtual Machine version

Works with: Euphoria 4.0.0

Another 'Paper tape' / Virtual Machine version, with as much as possible happening in the tape itself. Some command line handling as well.

<lang Euphoria> include std/mathcons.e

enum MUL_LLL, TESTEQ_LIL, TESTLT_LIL, TRUEGO_LL, MOVE_LL, INCR_L, TESTGT_LLL, GOTO_L, OUT_LI, OUT_II, STOP

global sequence tape = { 1, 1, 0, 0, 0, {TESTLT_LIL, 5, 0, 4}, {TRUEGO_LL, 4, 22}, {TESTEQ_LIL, 5, 0, 4}, {TRUEGO_LL, 4, 20}, {MUL_LLL, 1, 2, 3}, {TESTEQ_LIL, 3, PINF, 4}, {TRUEGO_LL, 4, 18}, {MOVE_LL, 3, 1}, {INCR_L, 2}, {TESTGT_LLL, 2, 5, 4 }, {TRUEGO_LL, 4, 18}, {GOTO_L, 10}, {OUT_LI, 3, "%.0f\n"}, {STOP}, {OUT_II, 1, "%.0f\n"}, {STOP}, {OUT_II, "Negative argument", "%s\n"}, {STOP} }

global integer ip = 1

procedure eval( sequence cmd ) atom i = 1 while i <= length( cmd ) do switch cmd[ i ] do case MUL_LLL then -- multiply location location giving location tape[ cmd[ i + 3 ] ] = tape[ cmd[ i + 1 ] ] * tape[ cmd[ i + 2 ] ] i += 3 case TESTEQ_LIL then -- test if location eq value giving location tape[ cmd[ i + 3 ]] = ( tape[ cmd[ i + 1 ] ] = cmd[ i + 2 ] ) i += 3 case TESTLT_LIL then -- test if location eq value giving location tape[ cmd[ i + 3 ]] = ( tape[ cmd[ i + 1 ] ] < cmd[ i + 2 ] ) i += 3 case TRUEGO_LL then -- if true in location, goto location if tape[ cmd[ i + 1 ] ] then ip = cmd[ i + 2 ] - 1 end if i += 2 case MOVE_LL then -- move value at location to location tape[ cmd[ i + 2 ] ] = tape[ cmd[ i + 1 ] ] i += 2 case INCR_L then -- increment value at location tape[ cmd[ i + 1 ] ] += 1 i += 1 case TESTGT_LLL then -- test if location gt location giving location tape[ cmd[ i + 3 ]] = ( tape[ cmd[ i + 1 ] ] > tape[ cmd[ i + 2 ] ] ) i += 3 case GOTO_L then -- goto location ip = cmd[ i + 1 ] - 1 i += 1 case OUT_LI then -- output location using format printf( 1, cmd[ i + 2], tape[ cmd[ i + 1 ] ] ) i += 2 case OUT_II then -- output immediate using format if sequence( cmd[ i + 1 ] ) then printf( 1, cmd[ i + 2], { cmd[ i + 1 ] } ) else printf( 1, cmd[ i + 2], cmd[ i + 1 ] ) end if i += 2 case STOP then -- stop abort(0) end switch i += 1 end while end procedure

include std/convert.e

sequence cmd = command_line() if length( cmd ) > 2 then puts( 1, cmd[ 3 ] & "! = " ) tape[ 5 ] = to_number(cmd[3]) else puts( 1, "eui fact.ex <number>\n" ) abort(1) end if

while 1 do if sequence( tape[ ip ] ) then eval( tape[ ip ] ) end if ip += 1 end while

</lang>

F#

This is a fast tail-recursive implementation. <lang fsharp>let factorial n : bigint =

   let rec f a n =
       match n with
       | 0I -> a
       | n -> (f (a * n) (n - 1I))
   f 1I n</lang>

> factorial 8I;;
val it : bigint = 40320I
> factorial 800I;;
val it : bigint = 771053011335386004144639397775028360595556401816010239163410994033970851827093069367090769795539033092647861224230677444659785152639745401480184653174909762504470638274259120173309701702610875092918816846985842150593623718603861642063078834117234098513725265045402523056575658860621238870412640219629971024686826624713383660963127048195572279707711688352620259869140994901287895747290410722496106151954257267396322405556727354786893725785838732404646243357335918597747405776328924775897564519583591354080898117023132762250714057271344110948164029940588827847780442314473200479525138318208302427727803133219305210952507605948994314345449325259594876385922128494560437296428386002940601874072732488897504223793518377180605441783116649708269946061380230531018291930510748665577803014523251797790388615033756544830374909440162270182952303329091720438210637097105616258387051884030288933650309756289188364568672104084185529365727646234588306683493594765274559497543759651733699820639731702116912963247441294200297800087061725868223880865243583365623482704395893652711840735418799773763054887588219943984673401051362280384187818611005035187862707840912942753454646054674870155072495767509778534059298038364204076299048072934501046255175378323008217670731649519955699084482330798811049166276249251326544312580289357812924825898217462848297648349400838815410152872456707653654424335818651136964880049831580548028614922852377435001511377656015730959254647171290930517340367287657007606177675483830521499707873449016844402390203746633086969747680671468541687265823637922007413849118593487710272883164905548707198762911703545119701275432473548172544699118836274377270607420652133092686282081777383674487881628800801928103015832821021286322120460874941697199487758769730544922012389694504960000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000I

Factor

Translation of: Haskell

<lang factor>USING: math.ranges sequences ;

factorial ( n -- n ) [1,b] product ;</lang>

The [1,b] word takes a number from the stack and pushes a range, which is then passed to product.

FALSE

<lang false>[1\[$][$@*\1-]#%]f: ^'0- f;!.</lang> Recursive: <lang false>[$1=~[$1-f;!*]?]f:</lang>

Fancy

<lang fancy>def class Number {

 def factorial {
   1 upto: self . product
 }

}

print first ten factorials

1 upto: 10 do_each: |i| {

 i to_s ++ "! = " ++ (i factorial) println

}</lang>

Fantom

The following uses 'Ints' to hold the computed factorials, which limits results to a 64-bit signed integer.

<lang fantom> class Main {

 static Int factorialRecursive (Int n)
 {
   if (n <= 1)
     return 1
   else
     return n * (factorialRecursive (n - 1))
 }

 static Int factorialIterative (Int n)
 {
   Int product := 1
   for (Int i := 2; i <=n ; ++i)
   {
     product *= i
   }
   return product
 }

 static Int factorialFunctional (Int n)
 {
   (1..n).toList.reduce(1) |a,v| 
   { 
     v->mult(a) // use a dynamic invoke
     // alternatively, cast a:  v * (Int)a
   }
 }

 public static Void main ()
 {
   echo (factorialRecursive(20))
   echo (factorialIterative(20))
   echo (factorialFunctional(20))
 }

} </lang>

Forth

Fortran

Works with: Fortran version 90 and later

A simple one-liner is sufficient

nfactorial = PRODUCT((/(i, i=1,n)/))

Works with: Fortran version 77 and later

<lang fortran> INTEGER FUNCTION FACT(N)

    INTEGER N, I
    FACT = 1
    DO 10, I = 1, N
      FACT = FACT * I

10 CONTINUE

    RETURN
    END</lang>

GAP

<lang gap># Built-in Factorial(5);

An implementation

fact := n -> Product([1 .. n]);</lang>

Genyris

<lang genyris>def factorial (n)

   if (< n 2) 1
     * n
       factorial (- n 1)</lang>

GML

<lang GML>n = argument0 j = 1 for(i = 1; i <= n; i += 1)

   j *= i

return j</lang>

Golfscript

Iterative (uses folding) <lang golfscript>{.!{1}{,{)}%{*}*}if}:fact; 5fact puts # test</lang> or <lang golfscript>{),(;{*}*}:fact;</lang>

Recursive <lang golfscript>{.1<{;1}{.(fact*}if}:fact;</lang>

Go

Iterative, sequential, but at least handling big numbers: <lang go> package main

import (

   "big"
   "fmt"

)

func main() {

   fmt.Println(factorial(800))

}

func factorial(n int64) *big.Int {

   if n < 0  {
       return nil
   }
   r := big.NewInt(1)
   var f big.Int
   for i := int64(2); i <= n; i++ {
       r.Mul(r, f.SetInt64(i))
   }
   return r

} </lang> Built in function currently uses a simple divide and conquer technique. It's a step up from sequential multiplication. <lang go> package main

import (

   "big"
   "fmt"

)

func factorial(n int64) *big.Int {

   var z big.Int
   return z.MulRange(1, n)

}

func main() {

   fmt.Println(factorial(800))

} </lang> For a bigger step up, an algorithm fast enough to compute factorials of numbers up to a million or so, see Factorial/Go.

Groovy

Recursive

A recursive closure must be pre-declared. <lang groovy>def rFact rFact = { (it > 1) ? it * rFact(it - 1) : 1 }</lang>

Test program: <lang groovy>(0..6).each { println "${it}: ${rFact(it)}" }</lang>

Output:

Iterative

<lang groovy>def iFact = { (it > 1) ? (2..it).inject(1) { i, j -> i*j } : 1 }</lang>

Test program: <lang groovy>(0..6).each { println "${it}: ${iFact(it)}" }</lang>

Output:

Haskell

The simplest description: factorial is the product of the numbers from 1 to n: <lang haskell>factorial n = product [1..n]</lang>

Or, written explicitly as a fold: <lang haskell>factorial n = foldl (*) 1 [1..n]</lang>

Or, if you wanted to generate a list of all the factorials: <lang haskell>factorials = scanl (*) 1 [1..]</lang>

Or, written without library functions:

<lang haskell> factorial :: Integral -> Integral factorial 0 = 1 factorial n = n * factorial (n-1) </lang>

HicEst

<lang hicest>WRITE(Clipboard) factorial(6) ! pasted: 720

FUNCTION factorial(n)

  factorial = 1
  DO i = 2, n
     factorial = factorial * i
  ENDDO

END</lang>

Icon and Unicon

Recursive

<lang Icon>procedure factorial(n)

  n := integer(n) | runerr(101, n)
  if n < 0 then fail
  return if n = 0 then 1 else n*factorial(n-1)

end </lang>

Iterative

The

Library: Icon Programming Library

factors provides the following iterative procedure which can be included with 'link factors':

<lang Icon>procedure factorial(n) #: return n! (n factorial)

  local i
  n := integer(n) | runerr(101, n)
  if n < 0 then fail
  i := 1
  every i *:= 1 to n
  return i

end</lang>

IDL

<lang idl>function fact,n

  return, product(lindgen(n)+1)

end</lang>

J

Operator

<lang j> ! 8 NB. Built in factorial operator 40320</lang>

Iterative / Functional

Recursive

Generalization

Factorial, like most of J's primitives, is generalized:

<lang j> ! 8 0.8 _0.8 NB. Generalizes as the gamma function 40320 0.931384 4.59084

 ! 800x          NB.  Also arbitrarily large

7710530113353860041446393977750283605955564018160102391634109940339708518270930693670907697955390330926478612242306774446597851526397454014801846531749097625044706382742591201733097017026108750929188168469858421505936237186038616420630788341172340985137252...</lang>

Java

Iterative

<lang java5>public static long fact(final int n) {

   if (n < 0) {
       System.err.println("No negative numbers");
       return 0;
   }
   long ans = 1;
   for (int i = 1; i <= n; i++) {
       ans *= i;
   }
   return ans;

}</lang>

Recursive

<lang java5>public static long fact(final int n) {

   if (n < 0){
       System.err.println("No negative numbers");
       return 0;
   }
   return (n < 2) ? 1 : n * fact(n - 1);

}</lang>

JavaScript

Several solutions are possible in JavaScript:

Iterative

<lang javascript>function factorial(n) {

   var x = 1;
   for (var i = 2; i <= n; i++) {
       x *= i;
   }
   return x;

}</lang>

Recursive

<lang javascript>function factorial(n) {

   return n < 2 ? 1 : n * factorial(n - 1);

}</lang>

Functional

Works with: JavaScript version 1.8

(See MDC)

<lang javascript>function factorial(n) {

 var nums = [1];
 for (var i=2; i<=n; i++) { // No built-in function to generate array of numbers a to n :(
   nums.push(n);
 }
 return nums.reduce(function(a, b) {return a*b});

}</lang>

Joy

<lang Joy> factorial = [zero?] [pop 1] [[] [1 -] bi factorial *] ifte </lang>

K

Iterative

 facti:*/1+!:
 facti 5

120 </lang>

Recursive

 factr:{:[x>1;x*_f x-1;1]}
 factr 6

720 </lang>

KonsolScript

<lang KonsolScript>function factorial(Number n):Number {

 Var:Number ret;
 if (n >= 0) {
   ret = 1;
   Var:Number i = 1;
   for (i = 1; i <= n; i++) {
     ret = ret * i;
   }
 } else {
   ret = 0;
 }
 return ret;

}</lang>

Korn Shell

Iterative

<lang korn>function factorial {

 typeset n=$1 f=1 i
 for ((i=2; i < n; i++)); do
   (( f *= i ))
 done
 echo $f

}</lang>

Recursive

<lang korn>function factorial {

 typeset n=$1
 (( n < 2 )) && echo 1 && return
 echo $(( n * $(factorial $((n-1))) ))

}</lang>

Liberty BASIC

<lang lb> for i =0 to 40

       print " FactorialI( "; using( "####", i); ") = "; factorialI( i)
       print " FactorialR( "; using( "####", i); ") = "; factorialR( i)
   next i

   wait

   function factorialI( n)
       if n >1 then
           f =1
           For i = 2 To n
               f = f * i
           Next i
       else
           f =1
       end if
   factorialI =f
   end function

   function factorialR( n)
       if n <2 then
           f =1
       else
           f =n *factorialR( n -1)
       end if
   factorialR =f
   end function

end

</lang>

Lisaac

<lang Lisaac>- factorial x : INTEGER : INTEGER <- (

 + result : INTEGER;
 (x <= 1).if {
   result := 1;
 } else {
   result := x * factorial(x - 1);
 };
 result

);</lang>

Logo

<lang logo>to factorial :n

 if :n < 2 [output 1]
 output :n * factorial :n-1

end</lang>

Lua

Recursive

<lang lua> function fact(n)

 return n > 0 and n * fact(n-1) or 1

end </lang>

Tail Recursive

<lang lua> function fact(n, acc)

 acc = acc or 1
 if n == 0 then
   return acc
 end
 return fact(n-1, n*acc)

end </lang>

M4

<lang M4>define(`factorial',`ifelse(`$1',0,1,`eval($1*factorial(decr($1)))')')dnl dnl factorial(5)</lang>

Output:

Mathematica

Note that Mathematica already comes with a factorial function, which can be used as e.g. 5! (gives 120). So the following implementations are only of pedagogical value.

Recursive

<lang mathematica>factorial[n_Integer] := n*factorial[n-1] factorial[0] = 1</lang>

Iterative (direct loop)

<lang mathematica>factorial[n_Integer] :=

 Block[{i, result = 1}, For[i = 1, i <= n, ++i, result *= i]; result]</lang>

Iterative (list)

factorial[n_Integer] := Block[{i}, Times @@ Table[i, {i, n}]]

MATLAB

Built-in

The factorial function is built-in to MATLAB. The built-in function is only accurate for N <= 21 due to the precision limitations of floating point numbers. <lang matlab>answer = factorial(N)</lang>

Recursive

<lang matlab>function f=fac(n)

   if n==0
       f=1;
       return
   else
       f=n*fac(n-1);
   end</lang>

Iterative

A possible iterative solution:

<lang matlab> function b=factorial(a) b=1; for i=1:a b=b*i; end</lang>

Maxima

Built-in

Recursive

Iterative

<lang maxima>fact2(n)::=block(r:1,for i: 1 thru n do r: r*i, done, r);</lang>

MAXScript

Iterative

<lang maxscript>fn factorial n = (

   if n == 0 then return 1
   local fac = 1
   for i in 1 to n do
   (
       fac *= i
   )
   fac

)</lang>

Recursive

<lang maxscript>fn factorial_rec n = (

   local fac = 1
   if n > 1 then
   (
       fac = n * factorial_rec (n - 1)
   )
   fac

)</lang>

ML/I

Iterative

<lang ML/I>MCSKIP "WITH" NL "" Factorial - iterative MCSKIP MT,<> MCINS %. MCDEF FACTORIAL WITHS () AS <MCSET T1=%A1. MCSET T2=1 MCSET T3=1 %L1.MCGO L2 IF T3 GR T1 MCSET T2=T2*T3 MCSET T3=T3+1 MCGO L1 %L2.%T2.> fact(1) is FACTORIAL(1) fact(2) is FACTORIAL(2) fact(3) is FACTORIAL(3) fact(4) is FACTORIAL(4)</lang>

Recursive

<lang ML/I>MCSKIP "WITH" NL "" Factorial - recursive MCSKIP MT,<> MCINS %. MCDEF FACTORIAL WITHS () AS <MCSET T1=%A1. MCGO L1 UNLESS T1 EN 0 1<>MCGO L0 %L1.%%T1.*FACTORIAL(%T1.-1).> fact(1) is FACTORIAL(1) fact(2) is FACTORIAL(2) fact(3) is FACTORIAL(3) fact(4) is FACTORIAL(4)</lang>

Modula-3

Iterative

<lang modula3>PROCEDURE FactIter(n: CARDINAL): CARDINAL =

 VAR
   result := n;
   counter := n - 1;
   
 BEGIN
   FOR i := counter TO 1 BY -1 DO
     result := result * i;
   END;
   RETURN result;
 END FactIter;</lang>

Recursive

<lang modula3>PROCEDURE FactRec(n: CARDINAL): CARDINAL =

 VAR result := 1;

 BEGIN
   IF n > 1 THEN
     result := n * FactRec(n - 1);
   END;
   RETURN result;
 END FactRec;</lang>

MUMPS

Iterative

<lang MUMPS> factorial(num) New ii,result If num<0 Quit "Negative number" If num["." Quit "Not an integer" Set result=1 For ii=1:1:num Set result=result*ii Quit result

Write $$factorial(0) ; 1 Write $$factorial(1) ; 1 Write $$factorial(2) ; 2 Write $$factorial(3) ; 6 Write $$factorial(10) ; 3628800 Write $$factorial(-6) ; Negative number Write $$factorial(3.7) ; Not an integer</lang>

Recursive

<lang MUMPS>factorial(num) ; If num<0 Quit "Negative number" If num["." Quit "Not an integer" If num<2 Quit 1 Quit num*$$factorial(num-1)

Write $$factorial(0) ; 1 Write $$factorial(1) ; 1 Write $$factorial(2) ; 2 Write $$factorial(3) ; 6 Write $$factorial(10) ; 3628800 Write $$factorial(-6) ; Negative number Write $$factorial(3.7) ; Not an integer</lang>

Nemerle

Here's two functional programming ways to do this and an iterative example translated from the C# above. Using long, we can only use number <= 20, I just don't like the scientific notation output from using a double. Note that in the iterative example, variables whose values change are explicitly defined as mutable; the default in Nemerle is immutable values, encouraging a more functional approach. <lang Nemerle>using System; using System.Console;

module Program {

 Main() : void
 {
     WriteLine("Factorial of which number?");
     def number = long.Parse(ReadLine());
     WriteLine("Using Fold : Factorial of {0} is {1}", number, FactorialFold(number));
     WriteLine("Using Match: Factorial of {0} is {1}", number, FactorialMatch(number));
     WriteLine("Iterative  : Factorial of {0} is {1}", number, FactorialIter(number));
 }
 
 FactorialFold(number : long) : long
 {
     $[1L..number].FoldLeft(1L, _ * _ )
 }
 
 FactorialMatch(number : long) : long
 {
     |0L => 1L
     |n  => n * FactorialMatch(n - 1L)
 }
 
 FactorialIter(number : long) : long
 {
     mutable accumulator = 1L;
     for (mutable factor = 1L; factor <= number; factor++)
     {
         accumulator *= factor;
     }
     accumulator  //implicit return
 }

}</lang>

newLISP

<lang newLISP>> (define (factorial n) (exp (gammaln (+ n 1)))) (lambda (n) (exp (gammaln (+ n 1)))) > (factorial 4) 24 </lang>

Nial

(from Nial help file) <lang nial>fact is recur [ 0 =, 1 first, pass, product, -1 +]</lang> Using it <lang nial>|fact 4 =24</lang>

Niue

Recursive

<lang Niue>[ dup 1 > [ dup 1 - factorial * ] when ] 'factorial ;

( test ) 4 factorial . ( => 24 ) 10 factorial . ( => 3628800 ) </lang>

Objeck

Iterative

<lang objeck> bundle Default {

 class Fact {
   function : Main(args : String[]) ~ Nil {
     5->Factorial()->PrintLine();
   }
 }

} </lang>

OCaml

Recursive

<lang ocaml>let rec factorial n =

 if n <= 0 then 1
 else n * factorial (n-1)</lang>

The following is tail-recursive, so it is effectively iterative: <lang ocaml>let factorial n =

 let rec loop i accum =
   if i > n then accum
   else loop (i + 1) (accum * i)
 in loop 1 1</lang>

Iterative

It can be done using explicit state, but this is usually discouraged in a functional language: <lang ocaml>let factorial n =

 let result = ref 1 in
 for i = 1 to n do
   result := !result * i
 done;
 !result</lang>

Octave

<lang octave>% built in factorial printf("%d\n", factorial(50));

% let's define our recursive... function fact = my_fact(n)

 if ( n <= 1 )
   fact = 1;
 else
   fact = n * my_fact(n-1);
 endif

endfunction

printf("%d\n", my_fact(50));

% let's define our iterative function fact = iter_fact(n)

 fact = 1;
 for i = 2:n
   fact = fact * i;
 endfor

endfunction

printf("%d\n", iter_fact(50));</lang>

Output:

30414093201713018969967457666435945132957882063457991132016803840
30414093201713375576366966406747986832057064836514787179557289984
30414093201713375576366966406747986832057064836514787179557289984

(Built-in is fast but use an approximation for big numbers)

Suggested correction: Neither of the three (two) results above is exact. The exact result (computed with Haskell) should be:

30414093201713378043612608166064768844377641568960512000000000000

In fact, all results given by Octave are precise up to their 16th digit, the rest seems to be "random" in all cases. Apparently, this is a consequence of Octave not being capable of arbitrary precision operation.

Oz

Folding

<lang oz>fun {Fac1 N}

  {FoldL {List.number 1 N 1} Number.'*' 1}

end</lang>

Tail recursive

<lang oz>fun {Fac2 N}

  fun {Loop N Acc}
     if N < 1 then Acc
     else

{Loop N-1 N*Acc}

     end
  end

in

  {Loop N 1}

end</lang>

Iterative

<lang oz>fun {Fac3 N}

  Result = {NewCell 1}

in

  for I in 1..N do
     Result := @Result * I
  end
  @Result

end</lang>

PARI/GP

All of these versions include bignum support. The recursive version is limited by the operating system's stack size; it may not be able to compute factorials larger than twenty thousand digits. The gamma function method is reliant on precision; to use it for large numbers increase default(realprecision) as needed. Moessner's algorithm is very slow but should be able to compute factorials until it runs out of memory (usage is about $n^{2}\log n$ bits to compute n!); a machine with 1 GB of RAM and unlimited time could, in theory, find 100,000-digit factorials.

Recursive

Iterative

This is an improvement on the naive recursion above, being faster and not limited by stack space. <lang parigp>fact(n)=my(p=1);for(k=2,n,p*=k);p</lang>

Binary splitting

PARI's prod automatically uses binary splitting, preventing subproducts from growing overly large. This function is dramatically faster than the above. <lang parigp>fact(n)=prod(k=2,n,k)</lang>

Built-in

Uses binary splitting. According to the source, this was found to be faster than prime decomposition methods. This is, of course, faster than the above. <lang parigp>fact(n)=n!</lang>

Gamma

Note also the presence of factorial and lngamma. <lang parigp>fact(n)=round(gamma(n+1))</lang>

Moessner's algorithm

Not practical, just amusing. Note the lack of * or ^. <lang parigp>fact(n)={

 my(v=vector(n+1,i,i==1));
 for(i=2,n+1,
   forstep(j=i,2,-1,
     for(k=2,j,v[k]+=v[k-1])
   )
 );
 v[n+1]

};</lang>

Pascal

Iterative

<lang pascal>function factorial(n: integer): integer;

var
 i, result: integer;
begin
 result := 1;
 for i := 2 to n do
  result := result * i;
 factorial := result
end;</lang>

Recursive

<lang pascal>function factorial(n: integer): integer;

begin
 if n = 0
  then
   factorial := 1
  else
   factorial := n*factorial(n-1)
end;</lang>

Perl

Iterative

<lang perl>sub factorial {

 my $n = shift;
 my $result = 1;
 for (my $i = 1; $i <= $n; ++$i)
 {
   $result *= $i;
 };
 $result;

}

using a .. range

sub factorial {

   my $r = 1;
   $r *= $_ for 1..shift;
   $r;

}</lang>

Recursive

<lang perl>sub factorial {

 my $n = shift;
 ($n == 0)? 1 : $n*factorial($n-1);

}</lang>

Functional

<lang perl>use List::Util qw(reduce); sub factorial {

 my $n = shift;
 reduce { $a * $b } 1, 1 .. $n

}</lang>

Perl 6

Works with: Rakudo version #22 "Thousand Oaks"

<lang perl6>sub postfix:<!> { [*] 1..$^n }

say 5!; #prints 120</lang>

PHP

Iterative

<lang php><?php function factorial($n) {

 if ($n < 0) {
   return 0;
 }

 $factorial = 1;
 for ($i = $n; $i >= 1; $i--) {
   $factorial = $factorial * $i;
 }

 return $factorial;

} ?></lang>

Recursive

<lang php><?php function factorial($n) {

 if ($n < 0) {
   return 0;
 }

 if ($n == 0) {
   return 1;
 }

 else {
   return $n * factorial($n-1);
 }

} ?></lang>

One-Liner

<lang php><?php function factorial($n) {

 return $n == 0 ? 1 : array_product(range(1, $n));

} ?></lang>

Library

Requires the GMP library to be compiled in: <lang php>gmp_fact($n)</lang>

PicoLisp

<lang PicoLisp>(de fact (N)

  (if (=0 N)
     1
     (* N (fact (dec N))) ) )</lang>

or: <lang PicoLisp>(de fact (N)

  (apply * (range 1 N) )</lang>

Piet

Codel width: 25

This is the text code. It is a bit difficult to write as there are some loops and loops doesn't really show well when I write it down as there is no way to explicitly write a loop in the language. I have tried to comment as best to show how it works <lang>push 1 not in(number) duplicate not // label a pointer // pointer 1 duplicate push 1 subtract push 1 pointer push 1 noop pointer duplicate // the next op is back at label a

push 1 // this part continues from pointer 1 noop push 2 // label b push 1 rot 1 2 duplicate not pointer // pointer 2 multiply push 3 pointer push 3 pointer push 3 push 3 pointer pointer // back at label b

pop // continues from pointer 2 out(number) exit</lang>

PL/I

<lang PL/I> factorial: procedure (N) returns (fixed decimal (30));

  declare N fixed binary nonassignable;
  declare i fixed decimal (10);
  declare F fixed decimal (30);

  if N < 0 then signal error;
  F = 1;
  do i = 2 to N;
     F = F * i;
  end;
  return (F);

end factorial; </lang>

PostScript

Recursive

<lang postscript>/fact {

 dup 0 eq     % check for the argument being 0
 {
   pop 1      % if so, the result is 1
 }
 {
   dup
   1 sub
   fact       % call recursively with n - 1
   mul        % multiply the result with n
 } ifelse

} def</lang>

Iterative

<lang postscript>/fact {

 1            % initial value for the product
 1 1          % for's start value and increment
 4 -1 roll    % bring the argument to the top as for's end value
 { mul } for

} def</lang>

Combinator

Library: initlib

<lang postscript> /myfact {{dup 0 eq} {pop 1} {dup pred} {mul} linrec}. </lang>

PowerShell

Recursive

<lang powershell>function Get-Factorial ($x) {

   if ($x -eq 0) {
       return 1
   }
   return $x * (Get-Factorial ($x - 1))

}</lang>

Iterative

<lang powershell>function Get-Factorial ($x) {

   if ($x -eq 0) {
       return 1
   } else {
       $product = 1
       1..$x | ForEach-Object { $product *= $_ }
       return $product
   }

}</lang>

Evaluative

Works with: PowerShell version 2

This one first builds a string, containing 1*2*3... and then lets PowerShell evaluate it. A bit of mis-use but works. <lang powershell>function Get-Factorial ($x) {

   if ($x -eq 0) {
       return 1
   }
   return (Invoke-Expression (1..$x -join '*'))

}</lang>

Prolog

Works with: SWI Prolog

Recursive

Tail recursive

<lang prolog>fact(N, NF) :- fact(1, N, 1, NF).

fact(X, X, F, F) :- !. fact(X, N, FX, F) :- FX1 is FX * X, X1 is X + 1, fact(X1, N, FX1, F). </lang>

Fold

We can simulate foldl. <lang prolog> % foldl(Pred, Init, List, R). % foldl(_Pred, Val, [], Val). foldl(Pred, Val, [H | T], Res) :- call(Pred, Val, H, Val1), foldl(Pred, Val1, T, Res).

% factorial p(X, Y, Z) :- Z is X * Y).

fact(X, F) :- numlist(2, X, L), foldl(p, 1, L, F).</lang>

Fold with anonymous function

Works with SWI-Prolog.
Using the module lambda written by Ulrich Neumerkel found there http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl, we can use anonymous functions and write : <lang prolog>

- use_module(lambda).

% foldl(Pred, Init, List, R). % foldl(_Pred, Val, [], Val). foldl(Pred, Val, [H | T], Res) :- call(Pred, Val, H, Val1), foldl(Pred, Val1, T, Res).

fact(N, F) :- numlist(2, N, L), foldl(\X^Y^Z^(Z is X * Y), 1, L, F).</lang>

Continuation passing style

Works with SWI-Prolog and module lambda written by Ulrich Neumerkel found there http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl. <lang prolog>:- use_module(lambda).

fact(N, FN) :- cont_fact(N, FN, \X^Y^(Y = X)).

cont_fact(N, F, Pred) :- ( N = 0 -> call(Pred, 1, F) ; N1 is N - 1,

P = \Z^T^(T is Z * N), cont_fact(N1, FT, P), call(Pred, FT, F) ). </lang>

Pure

Recursive

<lang pure>fact n = n*fact (n-1) if n>0;

      = 1 otherwise;

let facts = map fact (1..10); facts;</lang>

Tail Recursive

<lang pure>fact n = loop 1 n with

 loop p n = if n>0 then loop (p*n) (n-1) else p;

end;</lang>

PureBasic

Iterative

<lang PureBasic>Procedure factorial(n)

 Protected i, f = 1
 For i = 2 To n
   f = f * i
 Next
 ProcedureReturn f

EndProcedure</lang>

Recursive

<lang PureBasic>Procedure Factorial(n)

 If n < 2
   ProcedureReturn 1
 Else
   ProcedureReturn n * Factorial(n - 1)
 EndIf

EndProcedure</lang>

Python

Library

Works with: Python version 2.6+, 3.x

<lang python>import math math.factorial(n)</lang>

Iterative

<lang python>def factorial(n):

   result = 1
   for i in range(1, n+1):
       result *= i
   return result</lang>

Functional

<lang python>from operator import mul

def factorial(n):

   return reduce(mul, xrange(1,n+1), 1)</lang>

Sample output:

<lang python>>>> for i in range(6):

   print i, factorial(i)

0 1 1 1 2 2 3 6 4 24 5 120 >>></lang>

Numerical Approximation

The following sample uses Lanczos approximation from wp:Lanczos_approximation <lang python>from cmath import *

Coefficients used by the GNU Scientific Library

g = 7 p = [0.99999999999980993, 676.5203681218851, -1259.1392167224028,

    771.32342877765313, -176.61502916214059, 12.507343278686905,
    -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7]

def gamma(z):

 z = complex(z)
 # Reflection formula
 if z.real < 0.5:
   return pi / (sin(pi*z)*gamma(1-z))
 else:
   z -= 1
   x = p[0]
   for i in range(1, g+2):
     x += p[i]/(z+i)
   t = z + g + 0.5
   return sqrt(2*pi) * t**(z+0.5) * exp(-t) * x

def factorial(n):

 return gamma(n+1)

print "factorial(-0.5)**2=",factorial(-0.5)**2 for i in range(10):

 print "factorial(%d)=%s"%(i,factorial(i))</lang>

Output:

factorial(-0.5)**2= (3.14159265359+0j)
factorial(0)=(1+0j)
factorial(1)=(1+0j)
factorial(2)=(2+0j)
factorial(3)=(6+0j)
factorial(4)=(24+0j)
factorial(5)=(120+0j)
factorial(6)=(720+0j)
factorial(7)=(5040+0j)
factorial(8)=(40320+0j)
factorial(9)=(362880+0j)

Recursive

<lang python>def factorial(n):

   z=1
   if n>1:
       z=n*factorial(n-1)
   return z</lang>

Q

Iterative

Point-free

As a function

Recursive

R

Recursive

<lang R>fact <- function(n) {

 if ( n <= 1 ) 1
 else n * fact(n-1)

}</lang>

Iterative

<lang R>factIter <- function(n) {

 f = 1
 for (i in 2:n) f <- f * i
 f

}</lang>

Numerical Approximation

R has a native gamma function and a wrapper for that function that can produce factorials. E.g. <lang R>print(factorial(50)) # 3.041409e+64</lang>

REBOL

<lang REBOL>REBOL [

   Title: "Factorial"
   Author: oofoe
   Date: 2009-12-10
   URL: http://rosettacode.org/wiki/Factorial_function

]

Standard recursive implementation.

factorial: func [n][ either n > 1 [n * factorial n - 1] [1] ]

Iteration.

ifactorial: func [n][ f: 1 for i 2 n 1 [f: f * i] f ]

Automatic memoization.
I'm just going to say up front that this is a stunt. However, you've
got to admit it's pretty nifty. Note that the 'memo' function
works with an unlimited number of arguments (although the expected
gains decrease as the argument count increases).

memo: func [ "Defines memoizing function -- keeps arguments/results for later use." args [block!] "Function arguments. Just specify variable names." body [block!] "The body block of the function." /local m-args m-r ][ do compose/deep [ func [ (args) /dump "Dump memory." ][ m-args: [] if dump [return m-args]

if m-r: select/only m-args reduce [(args)] [return m-r]

m-r: do [(body)] append m-args reduce [reduce [(args)] m-r] m-r ] ] ]

mfactorial: memo [n][ either n > 1 [n * mfactorial n - 1] [1] ]

Test them on numbers zero to ten.

for i 0 10 1 [print [i ":" factorial i ifactorial i mfactorial i]]</lang>

Output:

0 : 1 1 1
1 : 1 1 1
2 : 2 2 2
3 : 6 6 6
4 : 24 24 24
5 : 120 120 120
6 : 720 720 720
7 : 5040 5040 5040
8 : 40320 40320 40320
9 : 362880 362880 362880
10 : 3628800 3628800 3628800

Retro

A recursive implementation from the benchmarking code.

<lang Retro>: <factorial> dup 1 = if; dup 1- <factorial> * ;

factorial dup 0 = [ 1+ ] [ <factorial> ] if ;</lang>

REXX

<lang REXX> /REXX program computes the factorial of argument. The program */ /* automatically adjusts the number of digits for large products.*/

numeric digits 100 /*start with 100 digits. */ parse arg x /*get the argument. */ if x= then call er 'no argument specified' if arg()>1 |,

  words(x)>1       then call er 'too many arguments specified.'

if \datatype(x,'N') then call er 'argument' x "must be numeric" if \datatype(x,'W') then call er 'argument' x "must be a whole number" if x<0 then call er 'argument' x "must not be negative"

!=1 /*define factorial produce so far.*/

 do j=2 to x                     /*compute factorial the hard way. */
 !=!*j                           /*multiple the factorial with J.  */
 if pos('E',!)==0 then iterate   /*is ! in exponential notation?   */
 parse var ! 'E' digs            /*pick off the factorial exponent.*/
 numeric digits digs+digs%10     /*  and incease it by ten percent.*/
 end

say x'! is ('length(!) "digits):" say !/1 /*normalize the factorial product.*/ exit

er: say; say '*** error! ***'; say arg(1); say; exit 13 </lang> Output:

(sample DOS prompt)

C:\►fact 1000
1000! is (2568 digits):
40238726007709377354370243392300398571937486421071463254379991042993851239862902
05920442084869694048004799886101971960586316668729948085589013238296699445909974
24504087073759918823627727188732519779505950995276120874975462497043601418278094
64649629105639388743788648733711918104582578364784997701247663288983595573543251
31853239584630755574091142624174743493475534286465766116677973966688202912073791
43853719588249808126867838374559731746136085379534524221586593201928090878297308
43139284440328123155861103697680135730421616874760967587134831202547858932076716
91324484262361314125087802080002616831510273418279777047846358681701643650241536
91398281264810213092761244896359928705114964975419909342221566832572080821333186
11681155361583654698404670897560290095053761647584772842188967964624494516076535
34081989013854424879849599533191017233555566021394503997362807501378376153071277
61926849034352625200015888535147331611702103968175921510907788019393178114194545
25722386554146106289218796022383897147608850627686296714667469756291123408243920
81601537808898939645182632436716167621791689097799119037540312746222899880051954
44414282012187361745992642956581746628302955570299024324153181617210465832036786
90611726015878352075151628422554026517048330422614397428693306169089796848259012
54583271682264580665267699586526822728070757813918581788896522081643483448259932
66043367660176999612831860788386150279465955131156552036093988180612138558600301
43569452722420634463179746059468257310379008402443243846565724501440282188525247
09351906209290231364932734975655139587205596542287497740114133469627154228458623
77387538230483865688976461927383814900140767310446640259899490222221765904339901
88601856652648506179970235619389701786004081188972991831102117122984590164192106
88843871218556461249607987229085192968193723886426148396573822911231250241866493
53143970137428531926649875337218940694281434118520158014123344828015051399694290
15348307764456909907315243327828826986460278986432113908350621709500259738986355
42771967428222487575867657523442202075736305694988250879689281627538488633969099
59826280956121450994871701244516461260379029309120889086942028510640182154399457
15680594187274899809425474217358240106367740459574178516082923013535808184009699
63725242305608559037006242712434169090041536901059339838357779394109700277534720
00000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000

C:\►

Ruby

Recursive

<lang ruby>def factorial_recursive(n)

 n.zero? ? 1 : n * factorial_recursive(n - 1)

end</lang>

Iterative

<lang ruby>def factorial_iterative(n)

  (2..n-1).each {|i| n*= i}
  n

end</lang>

Functional

<lang ruby>def factorial_functional(n)

 (1..n).reduce(1, :*)

end</lang>

Performance

<lang ruby>require 'benchmark'

n = 400 m = 10000

Benchmark.bm(12) do |b|

 b.report('recursive:')  {m.times {factorial_recursive(n)}}
 b.report('iterative:')  {m.times {factorial_iterative(n)}}
 b.report('functional:') {m.times {factorial_functional(n)}}

end</lang>

Output

                  user     system      total        real
recursive:    9.290000   0.040000   9.330000 (  9.364572)
iterative:    7.560000   0.020000   7.580000 (  7.612186)
functional:   7.030000   0.020000   7.050000 (  7.079359)

Sather

<lang sather>class MAIN is

 -- recursive
 fact(a: INTI):INTI is
   if a < 1.inti then return 1.inti; end;
   return a * fact(a - 1.inti);
 end;

 -- iterative
 fact_iter(a:INTI):INTI is
   s ::= 1.inti;
   loop s := s * a.downto!(1.inti); end;
   return s;
 end;

 main is
   a :INTI := 10.inti;
   #OUT + fact(a) + " = " + fact_iter(a) + "\n";
 end;

end;</lang>

Scala

Imperative

<lang scala>def factorial(n: Int)={

 var res = 1
 if(n > 0)
   for(i <- 1 to n)
     res *=i 
 res

} </lang>

Recursive

<lang scala>def factorial(n: Int): Int ={

 if(n == 0) 1
 else n * factorial(n-1)

} </lang>

Folding

<lang scala>def factorial(n: Int) = (2 to n).foldLeft(1)(_*_) </lang>

Using Pimp My Library pattern

<lang scala>// Note use of big integer support in this version

implicit def IntToFac(i : Int) = new {

 def ! = (2 to i).foldLeft(BigInt(1))(_*_)

}</lang>

Example use in the REPL:

scala> implicit def IntToFac(i : Int) = new {
     |   def ! = (2 to i).foldLeft(BigInt(1))(_*_)
     | }
IntToFac: (i: Int)java.lang.Object{def !: scala.math.BigInt}

scala> 20!
res0: scala.math.BigInt = 2432902008176640000

scala> 100!
res1: scala.math.BigInt = 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

Scheme

Recursive

<lang scheme>(define (factorial n)

 (if (<= n 0)
     1
     (* n (factorial (- n 1)))))</lang>

The following is tail-recursive, so it is effectively iterative: <lang scheme>(define (factorial n)

 (let loop ((i 1)
            (accum 1))
   (if (> i n)
       accum
       (loop (+ i 1) (* accum i)))))</lang>

Iterative

<lang scheme>(define (factorial n)

 (do ((i 1 (+ i 1))
      (accum 1 (* accum i)))
     ((> i n) accum)))</lang>

Folding

<lang scheme>;Using a generator and a function that apply generated values to a function taking two arguments

A generator knows commands 'next? and 'next

(define (range a b) (let ((k a)) (lambda (msg) (cond ((eq? msg 'next?) (<= k b)) ((eq? msg 'next) (cond ((<= k b) (set! k (+ k 1)) (- k 1)) (else 'nothing-left)))))))

Similar to List.fold_left in OCaml, but uses a generator

(define (fold fun a gen) (let aux ((a a)) (if (gen 'next?) (aux (fun a (gen 'next))) a)))

Now the factorial function

(define (factorial n) (fold * 1 (range 1 n)))

(factorial 8)

40320</lang>

Seed7

Seed7 defines the prefix operator ! , which computes a factorial of an integer. The maximum representable number for 32-bit signed integers is 2147483647. For 64-bit signed integers the maximum is 9223372036854775807. This limits the maximum factorial for 32-bit integers to factorial(12)=479001600 and for 64-bit integers to factorial(20)=2432902008176640000. Because of this limitations factorial is a very bad example to show the performance advantage of an iterative solution. To avoid this limitations the functions below use bigInteger:

Iterative

<lang seed7>const func bigInteger: factorial (in bigInteger: n) is func

 result
   var bigInteger: result is 1_;
 local
   var bigInteger: i is 0_;
 begin
   for i range 1_ to n do
     result *:= i;
   end for;
 end func;</lang>

Recursive

<lang seed7>const func bigInteger: factorial (in bigInteger: n) is func

 result
   var bigInteger: result is 1_;
 begin
   if n > 1_ then
     result := n * factorial(pred(n));
   end if;
 end func;</lang>

Sisal

Solution using a fold:

<lang sisal>define main

function main(x : integer returns integer)

 for a in 1, x
   returns
     value of product a
 end for

end function</lang>

Simple example using a recursive function:

<lang sisal>define main

function main(x : integer returns integer)

 if x = 0 then
   1
 else
   x * main(x - 1)
 end if

end function</lang>

Slate

This is already implemented in the core language as:

<lang slate>n@(Integer traits) factorial "The standard recursive definition." [

 n isNegative ifTrue: [error: 'Negative inputs to factorial are invalid.'].
 n <= 1
   ifTrue: [1]
   ifFalse: [n * ((n - 1) factorial)]

].</lang>

Here is another way to implement it:

<lang slate>n@(Integer traits) factorial2 [

 n isNegative ifTrue: [error: 'Negative inputs to factorial are invalid.'].
 (1 upTo: n by: 1) reduce: [|:a :b| a * b]

].</lang>

The output:

<lang slate>slate[5]> 100 factorial. 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000</lang>

Smalltalk

Smalltalk Number class already has a factorial method; however, let's see how we can implement it by ourselves.

Iterative with fold

Works with: GNU Smalltalk

<lang smalltalk>Number extend [

 my_factorial [
   (self < 2) ifTrue: [ ^1 ]
              ifFalse: [ |c|
                c := OrderedCollection new.
                2 to: self do: [ :i | c add: i ].

^ (c fold: [ :a :b | a * b ] )

              ]
 ]

].

7 factorial printNl. 7 my_factorial printNl.</lang>

Recursive

<lang smalltalk>Number extend [

 my_factorial [
   self < 0 ifTrue: [ self error: 'my_factorial is defined for natural numbers' ].
   self isZero ifTrue: [ ^1 ].
   ^self * ((self - 1) my_factorial)
 ]

].</lang>

Recursive (functional)

 |fac|

 fac := [:n |
   n < 0 ifTrue: [ self error: 'fac is defined for natural numbers' ].
   n <= 1 
       ifTrue: [ 1 ]
       ifFalse: [ n * (fac value:(n - 1)) ]
 ].
 fac value:1000.

].</lang>

SNOBOL4

Works with: Macro Spitbol

Works with: CSnobol

Note: Snobol4+ overflows after 7! because of signed short int limitation.

Recursive

<lang SNOBOL4> define('rfact(n)') :(rfact_end) rfact rfact = le(n,0) 1 :s(return)

       rfact = n * rfact(n - 1) :(return)

rfact_end</lang>

Tail-recursive

<lang SNOBOL4> define('trfact(n,f)') :(trfact_end) trfact trfact = le(n,0) f :s(return)

       trfact = trfact(n - 1, n * f) :(return)

trfact_end</lang>

Iterative

<lang SNOBOL4> define('ifact(n)') :(ifact_end) ifact ifact = 1 if1 ifact = gt(n,0) n * ifact :f(return)

       n = n - 1 :(if1)

ifact_end</lang>

Test and display factorials 0 .. 10

<lang SNOBOL4>loop i = le(i,10) i + 1 :f(end)

       output = rfact(i) ' ' trfact(i,1) ' ' ifact(i) :(loop)

end</lang>

Output:

1 1 1
2 2 2
6 6 6
24 24 24
120 120 120
720 720 720
5040 5040 5040
40320 40320 40320
362880 362880 362880
3628800 3628800 3628800
39916800 39916800 39916800

Standard ML

Recursive

<lang sml>fun factorial n =

 if n <= 0 then 1
 else n * factorial (n-1)</lang>

The following is tail-recursive, so it is effectively iterative: <lang sml>fun factorial n = let

 fun loop (i, accum) =
   if i > n then accum
   else loop (i + 1, accum * i)

in

 loop (1, 1)

end</lang>

Tcl

Works with: Tcl version 8.5

Use Tcl 8.5 for its built-in arbitrary precision integer support.

Iterative

<lang tcl>proc ifact n {

   for {set i $n; set sum 1} {$i >= 2} {incr i -1} {
       set sum [expr {$sum * $i}]
   }
   return $sum

}</lang>

Recursive

<lang tcl>proc rfact n {

   expr {$n < 2 ? 1 : $n * [rfact [incr n -1]]}

}</lang>

The recursive version is limited by the default stack size to roughly 850!

When put into the tcl::mathfunc namespace, the recursive call stays inside the expr language, and thus looks clearer:

<lang Tcl>proc tcl::mathfunc::fact n {expr {$n < 2? 1: $n*fact($n-1)}}</lang>

Iterative with caching

<lang tcl>proc ifact_caching n {

   global fact_cache
   if { ! [info exists fact_cache]} {
       set fact_cache {1 1}
   }
   if {$n < [llength $fact_cache]} {
       return [lindex $fact_cache $n]
   }
   set i [expr {[llength $fact_cache] - 1}]
   set sum [lindex $fact_cache $i]
   while {$i < $n} {
       incr i
       set sum [expr {$sum * $i}]
       lappend fact_cache $sum
   }
   return $sum

}</lang>

Performance Analysis

<lang tcl>puts [ifact 30] puts [rfact 30] puts [ifact_caching 30]

set n 400 set iterations 10000 puts "calculate $n factorial $iterations times" puts "ifact: [time {ifact $n} $iterations]" puts "rfact: [time {rfact $n} $iterations]"

for the caching proc, reset the cache between each iteration so as not to skew the results

puts "ifact_caching: [time {ifact_caching $n; unset -nocomplain fact_cache} $iterations]"</lang> Output:

265252859812191058636308480000000
265252859812191058636308480000000
265252859812191058636308480000000
calculate 400 factorial 10000 times
ifact: 661.4324 microseconds per iteration
rfact: 654.7593 microseconds per iteration
ifact_caching: 613.1989 microseconds per iteration

TI-89 BASIC

TI-89 BASIC also has the factorial function built in: x! is the factorial of x.

<lang ti89b>factorial(x) Func

 Return Π(y,y,1,x)

EndFunc</lang>

Π is the standard product operator: $\overbrace {\Pi (\mathrm {expr} ,i,a,b)} ^{\mathrm {TI-89} }=\overbrace {\prod _{i=a}^{b}\mathrm {expr} } ^{\text{Math notation}}$

UNIX Shell

Works with: bash

<lang bash>fact () {

 if [ $1 -eq 0 ];
   then echo 1;
   else echo $(($1 * $(fact $(($1-1)) ) ));
 fi;

}</lang>

Ursala

There is already a library function for factorials, but they can be defined anyway like this. The good method treats natural numbers as an abstract type, and the better method factors out powers of 2 by bit twiddling. <lang Ursala>#import nat

good_factorial = ~&?\1! product:-1^lrtPC/~& iota better_factorial = ~&?\1! ^T(~&lSL,@rS product:-1)+ ~&Z-~^*lrtPC/~& iota</lang> test program: <lang Ursala>#cast %nL

test = better_factorial* <0,1,2,3,4,5,6,7,8></lang> output:

<1,1,2,6,24,120,720,5040,40320>

VBScript

Optimized with memoization, works for numbers up to 170 (because of the limitations on Doubles), exits if -1 is input <lang VBScript>Dim lookupTable(170), returnTable(170), currentPosition, input currentPosition = 0

Do While True input = InputBox("Please type a number (-1 to quit):") MsgBox "The factorial of " & input & " is " & factorial(CDbl(input)) Loop

Function factorial (x) If x = -1 Then WScript.Quit 0 End If Dim temp temp = lookup(x) If x <= 1 Then factorial = 1 ElseIf temp <> 0 Then factorial = temp Else temp = factorial(x - 1) * x store x, temp factorial = temp End If End Function

Function lookup (x) Dim i For i = 0 To currentPosition - 1 If lookupTable(i) = x Then lookup = returnTable(i) Exit Function End If Next lookup = 0 End Function

Function store (x, y) lookupTable(currentPosition) = x returnTable(currentPosition) = y currentPosition = currentPosition + 1 End Function </lang>

Wrapl

Product

Recursive

Folding

x86 Assembly

Works with: nasm

Iterative

<lang asm>global factorial section .text

Input in ECX register (greater than 0!)
Output in EAX register

factorial:

 mov   eax, 1

.factor:

 mul   ecx
 loop  .factor
 ret</lang>

Recursive

<lang asm>global fact section .text

Input and output in EAX register

fact:

 cmp    eax, 1
 je    .done   ; if eax == 1 goto done

 ; inductive case
 push  eax  ; save n (ie. what EAX is)
 dec   eax  ; n - 1
 call  fact ; fact(n - 1)
 pop   ebx  ; fetch old n
 mul   ebx  ; multiplies EAX with EBX, ie. n * fac(n - 1)
 ret

.done:

 ; base case: return 1
 mov   eax, 1
 ret</lang>

Tail Recursive

<lang asm>global factorial section .text

Input in ECX register
Output in EAX register

factorial:

 mov   eax, 1  ; default argument, store 1 in accumulator

.base_case:

 test  ecx, ecx
 jnz   .inductive_case  ; return accumulator if n == 0
 ret

.inductive_case:

 mul   ecx         ; accumulator *= n
 dec   ecx         ; n -= 1
 jmp   .base_case  ; tail call</lang>

ZX Spectrum Basic

<lang zxbasic>10 LET x=5: GO SUB 1000: PRINT "5! = ";r 999 STOP 1000 REM ************* 1001 REM * FACTORIAL * 1002 REM ************* 1010 LET r=1 1020 IF x<2 THEN RETURN 1030 FOR i=2 TO x: LET r=r*i: NEXT i 1040 RETURN </lang>

Output:

5! = 120

@@ Line 1,523: / Line 1,523: @@
 =={{header|Maxima}}==
+=== Built-in ===
+<lang maxima>n!</lang>
 === Recursive ===
 <lang maxima>fact(n):=if n<2 then 1 else n*fact(n-1)</lang>
 === Iterative===
+<lang maxima>fact2(n)::=block(r:1,for i: 1 thru n do r: r*i, done, r);</lang>
-<lang maxima>
-fact2(n)::=block(r:1,for i: 1 thru n do r: r*i, done, r);
-</lang>
 =={{header|MAXScript}}==