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Re: sort order for strings of digits

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On 31 October 2012 23:09, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> The trick is to take each string and split it into a leading number and a
> trailing alphanumeric string. Either part may be "empty". Here's a pure
> Python solution:
>
> from sys import maxsize  # use maxint in Python 2
> def split(s):
>     for i, c in enumerate(s):
>         if not c.isdigit():
>             break
>     else:  # aligned with the FOR, not the IF
>         return (int(s), '')
>     return (int(s[:i] or maxsize), s[i:])
>
> Now sort using this as a key function:
>
> py> L = ['9', '1000', 'abc2', '55', '1', 'abc', '55a', '1a']
> py> sorted(L, key=split)
> ['1', '1a', '9', '55', '55a', '1000', 'abc', 'abc2']

You don't actually need to split the string, it's enough to return a
pair consisting of the number of leading digits followed by the string
as the key. Here's an implementation using takewhile:

>>> from itertools import takewhile
>>> def prefix(s):
...     return sum(1 for c in takewhile(str.isdigit, s)) or 1000, s
...
>>> L = ['9', '1000', 'abc2', '55', '1', 'abc', '55a', '1a']
>>> sorted(L, key=prefix)
['1', '1a', '9', '55', '55a', '1000', 'abc', 'abc2']

Here's why it works:

>>> map(prefix, L)
[(1, '9'), (4, '1000'), (1000, 'abc2'), (2, '55'), (1, '1'), (1000,
'abc'), (2, '55a'), (1, '1a')]

-- 
Arnaud

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sort order for strings of digits djc <djc@kangoo.invalid> - 2012-10-31 15:17 +0000
  Re: sort order for strings of digits Hans Mulder <hansmu@xs4all.nl> - 2012-10-31 16:31 +0100
  Re: sort order for strings of digits Ian Kelly <ian.g.kelly@gmail.com> - 2012-10-31 09:44 -0600
  Re: sort order for strings of digits Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-10-31 14:17 -0400
  Re: sort order for strings of digits Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-31 21:33 +0000
  Re: sort order for strings of digits Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-10-31 19:05 -0400
    Re: sort order for strings of digits Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-31 23:44 +0000
      Re: sort order for strings of digits Chris Angelico <rosuav@gmail.com> - 2012-11-01 11:53 +1100
        Re: sort order for strings of digits Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-02 00:27 +0000
  Re: sort order for strings of digits Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-31 23:09 +0000
    Re: sort order for strings of digits DJC <djc@news.invalid> - 2012-10-31 23:45 +0000
    Re: sort order for strings of digits Arnaud Delobelle <arnodel@gmail.com> - 2012-11-01 00:59 +0000
  Re: sort order for strings of digits Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-01 00:30 +0000
  Re: sort order for strings of digits wxjmfauth@gmail.com - 2012-11-01 01:52 -0700

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