Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!us.feeder.erje.net!news2.arglkargh.de!newsfeed.fsmpi.rwth-aachen.de!newsfeed.straub-nv.de!newsfeed.pionier.net.pl!feed.xsnews.nl!border03.ams.xsnews.nl!feeder04.ams.xsnews.nl!feeder03.ams.xsnews.nl!abp001.ams.xsnews.nl!frontend-F10-05.ams.news.kpn.nl
From: Cecil Westerhof <Cecil@decebal.nl>
Newsgroups: comp.lang.python
Subject: Re: Python is not bad ;-)
Organization: Decebal Computing
References: <87mw1q9jqw.fsf@Equus.decebal.nl> <mailman.121.1430385051.3680.python-list@python.org> <87383hj4zj.fsf@elektro.pacujo.net> <87d22lx38x.fsf@Equus.decebal.nl> <55432557$0$12994$c3e8da3$5496439d@news.astraweb.com>
X-Face: "(y8cC@tg_12{">GF'UXTW]FHI2wMiZNrnf'1EFQ&O#$m:f#O7+7}kR<J%a^F2lh4[N~Yz4 nSp#c+aQo1b5=?HcNEkQ7QzF<])O3X4MDL/AYjys&*mt>,v+Pti8=Vi/Z"g^?b"E
X-Homepage: http://www.decebal.nl/
Date: Sat, 02 May 2015 10:26:13 +0200
Message-ID: <87383fo062.fsf@Equus.decebal.nl>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
Cancel-Lock: sha1:K220dEjFrB2emYOUVcvKSUK47ng=
MIME-Version: 1.0
Content-Type: text/plain
Lines: 72
NNTP-Posting-Host: 81.207.62.244
X-Trace: 1430555331 news.kpn.nl 21118 81.207.62.244@kpn/81.207.62.244:36302
Xref: csiph.com comp.lang.python:89747

Op Friday 1 May 2015 09:03 CEST schreef Steven D'Aprano:

> On Thu, 30 Apr 2015 09:30 pm, Cecil Westerhof wrote:
>
>> Tail recursion would nice to have also.
>
> People coming from functional languages like Lisp and Haskell often
> say that, but how many recursive algorithms naturally take a
> tail-call form? Not that many.

One example:
    def factorial(x, y = 1):
        return y if x == 1 else factorial(x - 1, x * y)

    def factorial_iterative(x):
        result = 1
        for i in range(2, x + 1):
            result *= i
        return result

Executing factorial(985) 100.000 times takes 54 seconds.
While executing factorial_iterative(985) takes 34 seconds.
Also you can not call factorial with a value that is much bigger
because of recursion depth. You can call factorial_iterative with
1.000.000.

I made also a version that simulates tail recursion:
    def factorial_tail_recursion(x):
        y = 1
        while True:
            if x == 1:
                return y
            y *= x
            x -= 1

This is that a lot less efficient as the iterative version. It takes 43
seconds. But it is a lot better as the normal recursive version: about
25%. The iterative version is about 25% more efficient as the tail
recursion version.

With larger values it decreases. Calculating onetime for 5.000.000
takes 117 and 131 seconds. Just 10% faster.

That is mostly because the tail recursion version starts multiplying
at the high end. I wrote a second version:
    def factorial_tail_recursion2(x):
        y = 1
        z = 1
        while True:
            if x == z:
                return y
            y *= z
            z += 1

This has almost the performance of the iterative version: 34 and 121
seconds.

So I made a new recursive version:
    def factorial_recursive(x, y = 1, z = 1):
        return y if x == z else factorial_recursive(x, x * y, z + 1)

But this take almost the same tame as the other. Probably the
recursive calls are more important as the multiplications.


I find factorial a lot cleaner code as factorial_iterative, so here
tail recursion would be beneficial.

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