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From: Cecil Westerhof <Cecil@decebal.nl>
Newsgroups: comp.lang.python
Subject: Re: Throw the cat among the pigeons
Organization: Decebal Computing
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Date: Tue, 05 May 2015 21:42:52 +0200
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Op Tuesday 5 May 2015 20:45 CEST schreef Dave Angel:

> On 05/05/2015 12:18 PM, Cecil Westerhof wrote:
>
>>
>> Well, I did not write many tail recursive functions. But what
>> surprised me was that for large values the ‘tail recursive’ version
>> was more efficient as the iterative version. And that was with
>> myself implementing the tail recursion. I expect the code to be
>> more efficient when the compiler implements the tail recursion.
>>
>
>
> You've said that repeatedly, so I finally took a look at your
> webpage
>
> https://github.com/CecilWesterhof/PythonLibrary/blob/master/mathDecebal.py
>
> I didn't have your framework to call the code, so I just extracted
> some functions and did some testing.

I definitely need to take care of documentation.

It can be called with:
    python3 mathDecebal.py --factorial

The problem is that it will do correctness and speed test. I have to
split those in two different things. And use a different file for
both.

Maybe make a directory test and put a correctness_<function>.py and a
speed_<function>.py.



> I do see some differences,
> where the so-called tail_recursive functions are sometimes faster,
> but I did some investigating to try to determine why.
>
>
> I came up with the conclusion that sometimes the multiply operation
> takes longer than other times.  And in particular, i can see more
> variation between the two following loops than between your two
> functions.
>
>
> def factorial_iterative(x, simple=False):
> assert x >= 0
> result = 1
> j=2
> if not simple:
> for i in range(2, x + 1):
> result *= i
> j += 1
> else:
> for i in range(2, x + 1):
> result *= j
> j += 1
> pass
>
> return result
>
> When the "simple" is True, the function takes noticeably and
> consistently longer.  For example, it might take 116 instead of 109
> seconds.  For the same counts, your code took 111.
>
> I've looked at dis.dis(factorial_iterative), and can see no explicit
> reason for the difference.

I would say that a variable that is filled by a range is different as
a normal variable. Do not ask me why. ;-)

Even if you (general not personal you) think that the tail recursion
is a waist of time, this is an interesting result I think.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof