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Re: Python is not bad ;-)

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Op Saturday 2 May 2015 12:35 CEST schreef Dave Angel:

> On 05/02/2015 05:33 AM, Cecil Westerhof wrote:
>
> Please check your email settings. Your messages that you type seem
> to be indented properly, but those that are quoting earlier messages
> (even your own) are not. See below. I suspect there's some problem
> with how your email program processes html messages.
>
>> Op Saturday 2 May 2015 10:26 CEST schreef Cecil Westerhof:
>>>
>>> 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)
>>
>> Stupid me 'x == z' should be 'z > x'
>>
>
> I can't see how that is worth doing. The recursive version is
> already a distortion of the definition of factorial that I learned.
> And to force it to be recursive and also contort it so it does the
> operations in the same order as the iterative version, just to gain
> performance?
>
> If you want performance on factorial, write it iteratively, in as
> straightforward a way as possible. Or just call the library
> function.
>
> Recursion is a very useful tool in a developer's toolbox. But the
> only reason I would use it for factorial is to provide a simple
> demonstration to introduce the concept to a beginner.

And that was what I was doing here: showing that tail recursion can
have benefits.

By the way I have seen factorial mostly implemented recursive.

Also I am now testing with very large values. The version where I use
both tail recursion versions are faster as the iterative version.
Calculating 500.000:
iterative:                  137 seconds
tail recursion optimised:   120 seconds
tail recursion simple:      130 seconds

You have to be careful that you do not fall into the trap that you are
sure your way is the best way. I have had the same on the Scala list.
I told there that for a certain function it was better to use an
iterative version as a tail recursive function. They did not want to
believe it at first.

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

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Thread

Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 09:07 +0200
  Re: Python is not bad ;-) Ben Finney <ben+python@benfinney.id.au> - 2015-04-30 19:10 +1000
    Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-04-30 13:16 +0300
      Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-04-30 20:52 +1000
      Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 13:30 +0200
        Re: Python is not bad ;-) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-01 17:03 +1000
          Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-01 09:47 +0200
          Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-01 19:56 +0200
          Re: Python is not bad ;-) Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-05-02 19:44 +1200
          Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 10:26 +0200
            Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 12:10 +0300
              Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 19:25 +1000
                Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 12:58 +0300
                Re: Python is not bad ;-) Dave Angel <davea@davea.name> - 2015-05-02 06:22 -0400
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 20:42 +1000
                Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-02 13:07 +0200
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-02 21:21 +1000
                Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-05-02 13:32 +0200
                Re: Python is not bad ;-) Marko Rauhamaa <marko@pacujo.net> - 2015-05-02 14:42 +0300
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 09:45 -0600
                Re: Python is not bad ;-) Chris Angelico <rosuav@gmail.com> - 2015-05-03 01:55 +1000
                Re: Python is not bad ;-) Joonas Liik <liik.joonas@gmail.com> - 2015-05-02 16:50 +0300
                Re: Python is not bad ;-) Joonas Liik <liik.joonas@gmail.com> - 2015-05-02 18:53 +0300
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 11:00 -0600
                Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 11:17 -0600
                Re: Python is not bad ;-) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-05-02 18:22 +0100
              Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 12:29 +0200
            Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 11:33 +0200
              Re: Python is not bad ;-) Dave Angel <davea@davea.name> - 2015-05-02 06:35 -0400
                Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-05-02 13:12 +0200
              Re: Python is not bad ;-) Ian Kelly <ian.g.kelly@gmail.com> - 2015-05-02 09:31 -0600
      Re: Python is not bad ;-) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-01 15:56 +1000
    Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 13:10 +0200
  Re: Python is not bad ;-) Michael Torrie <torriem@gmail.com> - 2015-04-30 08:03 -0600
    Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 18:11 +0200
      Re: Python is not bad ;-) Christian Gollwitzer <auriocus@gmx.de> - 2015-04-30 19:59 +0200
        Re: Python is not bad ;-) Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 22:05 +0200

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