From: James Cunningham <jameshcunningham@...>
Date: 2006-11-27T09:05:07+09:00
Subject: Re: ruby and list comprehension

On 2006-11-26 19:01:53 -0500, James Cunningham 
<jameshcunningham@gmail.com> said:

> 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

Er, "your" meaning "Brad Tilley's".

Best,
James