From: James Cunningham <jameshcunningham@...>
Date: 2006-11-27T09:05:06+09:00
Subject: Re: ruby and list comprehension

On 2006-11-26 17:52:37 -0500, dblack@wobblini.net said:

> 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


I should have clarified. In your example there's no difference, but the 
above gives a general replacement for list comprehensions.

irb(main):018:0> (1..25).to_a.comprehend { |x| x**2 if not x % 2 == 0 }
=> [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

Best,
James