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Re: please i need explanation

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  Re: please i need explanation kwakukwatiah@gmail.com - 2013-01-11 10:33 -0600
    Re: please i need explanation Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-01-11 14:20 +0200
    Re: please i need explanation Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2013-01-11 13:58 +0100

#36614 — Re: please i need explanation

-----Original Message----- 
From: K. Elo
Sent: Friday, January 11, 2013 3:56 AM
To: python-list@python.org
Subject: Re: please i need explanation

Hi!

Since there is no stated question, I need to guess:

n -= 1 (instead of "f -= 1")

should work.

Or maybe the question was a totally different one...

-Kimmo

11.01.2013 17:35, kwakukwatiah@gmail.com wrote:
> def factorial(n):
>      if n<2:
>               return 1
>      f = 1
>      while n>= 2:
>          f *= n
>          f -= 1
>      return f
>
>
>
please it works.but don’t get why the return 1 and the code below.

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#36617

kwakukwatiah@gmail.com writes:
> 11.01.2013 17:35, kwakukwatiah@gmail.com wrote:
> > def factorial(n):
> >      if n<2:
> >               return 1
> >      f = 1
> >      while n>= 2:
> >          f *= n
> >          f -= 1
> >      return f
>
> please it works.but don’t get why the return 1 and the code below.

Ignoring the error that has been pointed out, this code seems to be
"optimized" to avoid multiplication by 1. I doubt if the difference is
measurable. If anyone cares enough to measure:

def fast_factorial(n):
   if n < 2: return 1
   f = 1
   while n > 1:
     f *= n
     n -= 1
   return f

def slow_factorial(n):
   f = 1
   while n != 0:
     f *= n
     n -= 1
   return f

(Untested, and just kidding. For fast factorial routines, search for a
paper by Richard Fateman on the web. They're in Common Lisp, but the
branching techniques should be generally applicable.)

Additionally, your code produces nonsense quietly when called with
(real) numbers outside its intended domain (negative integers and
non-integers are outside). Not a good idea. (My version will loop
indefinitely instead. Better than a wrong answer but also not very
good. Caller beware. :)

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#36619

Am 11.01.2013 17:33 schrieb kwakukwatiah@gmail.com:

>> def factorial(n):
>>      if n<2:
>>               return 1
>>      f = 1
>>      while n>= 2:
>>          f *= n
>>          f -= 1
>>      return f
>>
>>
>>
> please it works.

I doubt this.

If you give n = 4, you run into an endless loop.


> but don’t get why the return 1 and the code below.

The "if n < 2: return 1" serves to shorten the calculation process 
below. It is redundant, as you have a "f = 1" and a "return f" for n < 2.

The code below first sets f, which holds the result, to 1 and then 
multiplies it by n in each step. As the loop should contain a 'n -= 1', 
n decreases by 1 every step, turning it into f = n * (n-1) * (n-2) * ... 
* 2 and then, as n is not >= 2 any longer, stops the loop, returning f.

HTH,

Thomas

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