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Groups > comp.lang.python > #99948

Re: filter a list of strings

From Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
Newsgroups comp.lang.python
Subject Re: filter a list of strings
Date 2015-12-03 10:46 +0100
Message-ID <mailman.167.1449136035.14615.python-list@python.org> (permalink)
References <mailman.155.1449122975.14615.python-list@python.org> <lf51tb459f2.fsf@ling.helsinki.fi> <3pBBdn3nVcz5vP8@dovecot03.posteo.de>

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On 03.12.2015 10:27, c.buhtz@posteo.jp wrote:
 >
 > I often saw constructions like this
 >    x for x in y if ...
 > But I don't understand that combination of the Python keywords (for,
 > in, if) I allready know. It is to complex to imagine what there really
 > happen.
 >
 > I understand this
 >    for x in y:
 >      if ...
 >
 > But what is about the 'x' in front of all that?
 >

The leading x states which value you want to put in the new list. This 
may seem obvious in the simple case, but quite often its not the 
original x-ses found in y that you want to store, but some 
transformation of it, e.g.:

[x**2 for x in y]

is equivalent to:

squares = []
for x in y:
     squares.append(x**2)

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Thread

filter a list of strings <c.buhtz@posteo.jp> - 2015-12-03 02:15 +0100
  Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 08:32 +0200
    Re: filter a list of strings <c.buhtz@posteo.jp> - 2015-12-03 10:27 +0100
      Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 13:53 +0200
      Re: filter a list of strings Peter Pearson <pkpearson@nowhere.invalid> - 2015-12-05 19:42 +0000
    Re: filter a list of strings Chris Angelico <rosuav@gmail.com> - 2015-12-03 20:40 +1100
    Re: filter a list of strings Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2015-12-03 10:46 +0100
    Re: filter a list of strings Laura Creighton <lac@openend.se> - 2015-12-03 10:53 +0100
    Re: filter a list of strings jmp <jeanmichel@sequans.com> - 2015-12-03 11:03 +0100
    Re: filter a list of strings Peter Otten <__peter__@web.de> - 2015-12-03 11:13 +0100
    Re: filter a list of strings Denis McMahon <denismfmcmahon@gmail.com> - 2015-12-03 14:16 +0000
      Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 17:02 +0200
  Re: filter a list of strings Grobu <snailcoder@retrosite.invalid> - 2015-12-03 13:17 +0100

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