Important Shock

4 Comments Add your own

• 1. Peter Hosey  |  December 25, 2006 at 7:24 pm

  Here’s an alternate solution, unless you count the import operator statement…

  def fact(x): return reduce(operator.mul, xrange(2, x+1))

  If you want to be really strict about it, you can do this:

  def fact(x): return reduce(lambda x, y: x*y, xrange(2, x+1))

  but it probably won’t be as fast, since you’re now using a Python function in place of a built-in.

  As for exactly how these two functions stack up against yours, here’s what I get on my Mac Pro:

  >>> time_ptfact(500, 5000)
  0.00046299901008605955
  >>> time_reducefact(500, 5000)
  0.00028626961708068849
  >>> time_reducefact_lambda(500, 5000)
  0.0003613057613372803

  Shows once again the value of doing as much as possible in C code (reduce, operator.mul, and xrange all being built-ins).

  The timing functions are defined as:

  def time_FUNC(x, count=500):
  import time
  start = time.time()
  for i in xrange(count):
  FUNC(x) and None
  end = time.time()
  return (end - start) / count

  for all three values of FUNC.

• 2. Aditya  |  June 3, 2008 at 6:44 am

  Thanks for the information Peter, but can’t you just give the command ‘time’ while calculating the factorial? I mean, give the command,

  time python programName.py

  it will output the time. Built-in time function for all *nix systems.

• 3. Michael Hartley  |  June 25, 2008 at 7:23 am

  err.. pardon my ignorance of large non-venomous snakes, but how do I actually use this? Here’s what happened when I tried :

  [mikeh@pud21 ~]$ python
Python 2.4.3 (#1, Mar 14 2007, 19:01:42)
[GCC 4.1.1 20070105 (Red Hat 4.1.1-52)] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> def fact(x): return (1 if x==0 else x * fact(x-1))
File “”, line 1
def fact(x): return (1 if x==0 else x * fact(x-1))
^
SyntaxError: invalid syntax
>>>

• 4. Jameson  |  June 26, 2008 at 7:50 am

  python needs a carriage returns followed by an indentation after defining a function… do it like this:

  
  >>> def fact(x):
  … return (1 if x==0 else x * fact(x-1))
  …
  >>> fact(1)
  1
  >>> fact(10)
  3628800
  

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