From: Gregory Seidman <gsslist+ruby@...>
Date: 2008-10-31T07:31:01+09:00
Subject: Re: how to sort this array

On Fri, Oct 31, 2008 at 06:14:51AM +0900, Li Chen wrote:
> Hi,
> 
> I have an array containing files names. How do I sort them so that I can
> get the expected results?
> 
> Thanks,
> 
> Li
> 
> #############
> files=[
>   "c:/ruby/self/2004/20.txt",
>   "c:/ruby/self/2004/3.txt",
>   "c:/ruby/self/2004/2.txt",
>   "c:/ruby/self/2004/10.txt",
>   "c:/ruby/self/2004/1.txt"
> ]
> 
> expected results:
> [
> "c:/ruby/self/2004/1.txt",
> "c:/ruby/self/2004/2.txt",
> "c:/ruby/self/2004/3.txt",
> "c:/ruby/self/2004/10.txt",
> ]
> "c:/ruby/self/2004/20.txt",

files = files.sort_by { |f| f[/\/(\d+)\.[^\/]*\Z/, 1].to_i }

There are several things to explain here:

- The #sort_by method calls the block on each element to produce a set of
  surrogate values to use as sorting keys.

- The regular expression captures the series of digits between the last
  slash in a string and the following dot.

- The [] method can be called on String in many ways, including passing a
  RegExp and an index. That form returns the capture of the appropriate
  index from matching the RegExp or nil if the RegExp does not match.

- Calling #to_i on the result means that it will either parse the captured
  sequence of digits as an integer or, if the RegExp fails, return 0.

--Greg