From: Robert Klemme <shortcutter@...>
Date: 2010-02-19T23:16:25+09:00
Subject: Re: Adding self to Enumerable

2010/2/19 Glenn Ritz <glenn_ritz@yahoo.com>:
> Robert Klemme wrote:
>> On 02/18/2010 10:39 PM, Glenn Ritz wrote:
>>>>
>>>> %w(cat dog mouse).each { |e| s = self; puts "self: #{s.inspect}, e:
>>>> #{e}" }
>>>>
>>>> or better:
>>>>
>>>> %w(cat dog mouse).each { |e| puts "self: #{self.inspect}, e: #{e}" }
>>>
>>>
>>> I want to be able to refer to the array receiver inside of the block.
>>
>> What is the point of that?  I mean, you can *always* have a reference to
>> the instance around.
>>
>> s = %w(cat dog mouse)
>> s.each {|e| ...}
>>
>> Or, in 1.9
>>
>> %w(cat dog mouse).tap {|s| s.each {|e| ...}}
>>
>> Even if you pass around a block for later usage with #each you can make
>> sure the collection is accessible inside the block.
>>
>> What is the real use case of this?  Passing the collection into the
>> block during iteration feels wrong.  For example, typically when
>> iterating modifications of the underlying collection are dangerous.  Why
>> do you need this feature?
>
> Sometimes in statistics it's good to be able to access the previous
> element in a collection, not just the current one like in Ruby's each
> method.  So I was thinking that it would be good if I could refer to the
> array itself within the block.  I realize now that there are probably
> better ways to do this. :-)

Enumerable#each_cons comes to mind...

Also, Enumerable#inject works pretty well:

irb(main):003:0> (1..5).inject {|a,b| p [a,b];b}
[1, 2]
[2, 3]
[3, 4]
[4, 5]
=> 5
irb(main):004:0> (1..5).inject(nil) {|a,b| p [a,b];b}
[nil, 1]
[1, 2]
[2, 3]
[3, 4]
[4, 5]
=> 5
irb(main):005:0>

> Also, I find the with_index method on Enumerable objects interesting and
> was trying to figure out a way to write a similar method.  So I asked
> the question just to learn more on what I think is an interesting
> feature of the language.

I hope you found out.

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/