Function for Factorial in Python

Question

How do I go about computing a factorial of an integer in Python

User · Answer

Non-recursive solution  no imports  def factorial x       return eval       join map str  range 1  x   1

User · Answer

def fact n       f   1     for i in range 1  n   1           f    i     return f

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If you are using Python2 5 or older try  from operator import mul def factorial n       return reduce mul  range 1 n 1     for newer Python  there is factorial in the math module as given in other answers here

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Easiest way is to use math factorial  available in Python 2 6 and above    import math math factorial 1000    If you want have to write it yourself  you can use an iterative approach   def factorial n       fact   1     for num in range 2  n   1           fact    num     return fact   or a recursive approach   def factorial n       if n  lt  2          return 1     else          return n   factorial n-1    Note that the factorial function is only defined for positive integers so you should also check that n  gt   0 and that isinstance n  int   If it s not  raise a ValueError or a TypeError respectively  math factorial will take care of this for you

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Another way to do it is to use np prod shown below   def factorial n       if n    0          return 1     else           return np prod np arange 1 n 1

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def factorial n       if n  lt  2          return 1     return n   factorial n - 1

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Existing solution  The shortest and probably the fastest solution is   from math import factorial print factorial 1000    Building your own  You can also build your own solution  Generally you have two approaches  The one that suits me best is   from itertools import imap def factorial x       return reduce long   mul    imap long  xrange 1  x   1     print factorial 1000     it works also for bigger numbers  when the result becomes long   The second way of achieving the same is   def factorial x       result   1     for i in xrange 2  x   1           result    i     return result  print factorial 1000

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For performance reasons  please do not use recursion  It would be disastrous   def fact n  total 1       while True          if n    1              return total         n  total   n - 1  total   n       Check running results  cProfile run  fact 126000        4 function calls in 5 164 seconds   Using the stack is convenient like recursive call   but it comes at a cost  storing detailed information can take up a lot of memory   If the stack is high  it means that the computer stores a lot of information about function calls   The method only takes up constant memory like iteration        Or Using for loop  def fact n       result   1     for i in range 2  n   1           result    i     return result       Check running results  cProfile run  fact 126000        4 function calls in 4 708 seconds         Or Using builtin function math  def fact n       return math factorial n        Check running results  cProfile run  fact 126000        5 function calls in 0 272 seconds

User · Answer

On Python 2 6 and up  try   import math math factorial n

[python] Function for Factorial in Python

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