From: dblack@...
Date: 2006-11-27T07:52:37+09:00
Subject: Re: ruby and list comprehension

Hi --

On Mon, 27 Nov 2006, James Cunningham wrote:

> On 2006-11-25 18:47:26 -0500, Brad Tilley <rtilley@vt.edu> said:
>
>> In Python, I can do this to arrays:
>> 
>> added = [x for x in new_data if x not in old_data]
>> removed = [x for x in old_data if x not in new_data]
>> same = [x for x in new_data if x in old_data]
>> 
>> I believe this is known as list comprehension in Python. How is this done 
>> in
>> Ruby?
>> 
>> Thanks,
>> Brad
>
> I found this while web searching for the same thing recently; I can't recall 
> where I found it. It's a cute little hack.
>
> class Array
> def comprehend
>   return self unless block_given?
>   result = []
>   self.each { |i| result.push yield(i) }
>   result.compact
> end
> end
>
> Then:
>
> added = new_data.comprehend { |x| x if not old_data.include? x }
> removed = old_data.comprehend { |x| x if not new_data.include? x }
> same = new_data.comprehend { |x| x if old_data.include? x }

I'm not getting how that's better than:

   added = new_data.select {|x| not old_data.include?(x) }

(or the reject equivalent) and so on.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org