Re: Help with map python 2

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From	Terry Reedy <tjreedy@udel.edu>
Subject	Re: Help with map python 2
Date	Mon, 05 Jan 2015 18:28:40 -0500
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On 1/5/2015 6:33 AM, flebber wrote:
>
>> You could do what mathematicians do when they deal with alternating
>> signs: they raise -1 to the power of the index to get an appropriate
>> multiplier.
>>
>>     >>> [ n * (-1) ** n for n in range(10) ]
>>     [0, -1, 2, -3, 4, -5, 6, -7, 8, -9]

Mathematicians operating in the timeless Platonic universe of 
mathemtical forms are not concerned with such droll issues as 
computation time.  I think most regard '(-1)**n' as an abbreviation for 
'(-1 if odd(n) else 1)', where odd(n) is regarded as an O(1) operation, 
not as an indication to actually raise -1 to an arbitrarily large power. 
which could take an arbitrarily long time.


>> Or you could do here what you attempt to do with map below. See below.
>>
>
>>
>> You are trying to use a binary expression. There are no binary
>> expressions. Add an else branch to make it ternary:
>>
>>     lambda x : x if x % 2 == 0 else -x
>>
>> But never mind the number of branches, the serious point is that you
>> didn't specify a value for when the condition is not true. It doesn't
>> make sense without that.
>>
>> There's nothing wrong with a list comprehension, or the corresponding
>> generator expression if you want a generator. It's fine. Map's fine.
>
> Thanks Jussi  I really like the multiplier solution.
>


-- 
Terry Jan Reedy

Thread

Help with map python 2 flebber <flebber.crue@gmail.com> - 2015-01-04 03:34 -0800
  Re: Help with map python 2 Chris Angelico <rosuav@gmail.com> - 2015-01-04 22:47 +1100
  Re: Help with map python 2 Alec Taylor <alec.taylor6@gmail.com> - 2015-01-04 22:47 +1100
  Re: Help with map python 2 Alec Taylor <alec.taylor6@gmail.com> - 2015-01-04 22:48 +1100
  Re: Help with map python 2 Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2015-01-04 14:08 +0200
    Re: Help with map python 2 flebber <flebber.crue@gmail.com> - 2015-01-05 03:33 -0800
      Re: Help with map python 2 Terry Reedy <tjreedy@udel.edu> - 2015-01-05 18:28 -0500
        Re: Help with map python 2 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-06 12:30 +1100
  Re: Help with map python 2 Peter Otten <__peter__@web.de> - 2015-01-04 17:33 +0100
  Re: Help with map python 2 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-01-04 12:32 -0500
  Re: Help with map python 2 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-01-04 12:41 -0500
  Re: Help with map python 2 Dan Stromberg <drsalists@gmail.com> - 2015-01-04 10:06 -0800
  Re: Help with map python 2 Terry Reedy <tjreedy@udel.edu> - 2015-01-04 16:13 -0500
    Re: Help with map python 2 flebber <flebber.crue@gmail.com> - 2015-01-05 03:37 -0800
  Re: Help with map python 2 Sayth Renshaw <flebber.crue@gmail.com> - 2015-01-04 21:22 +0000

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