From: Gavin Kistner <gavin@...>
Date: 2005-04-22T23:11:52+09:00
Subject: Re: for .. in .. else?

On Apr 22, 2005, at 7:34 AM, Adam Keys wrote:
> Maybe I missed something, but when I extracted this code and ran it in 
> irb, it didn't work like I thought it would:

It should have been 'if' instead of 'unless'.

Or, make it reusable:

class Array
	def each_or_empty( options = {} )
		if self.empty?
			empty_block = options['when_empty'] || options[:when_empty] || 
options['empty'] || options[:empty]
			empty_block.call() if empty_block
		else
			each_block = options['each'] || options[:each]
			self.each{ |item| each_block.call( item ) } if each_block
		end
		self
	end
end

some_songs = %w| song1 song2 song3 |
no_songs = []

[some_songs, no_songs].each_with_index do |collection, i|
	puts "Checking collection ##{i}"
	collection.each_or_empty :each => lambda{ |song| p song }, 
:when_empty=>lambda{ p 'no songs!' }
end
puts "...done"

#=> Checking collection #0
#=> "song1"
#=> "song2"
#=> "song3"
#=> Checking collection #1
#=> "no songs!"
#=> ...done


>
> irb(main):087:0> songs
> => ["Rocky Raccoon", "As My Guitar Gently Weeps", "She Came In Through 
> The Bathroom Window"]
> irb(main):088:0> unless songs.each do |song|
> irb(main):089:2* p song
> irb(main):090:2> end.empty?
> irb(main):091:1> p 'no songs!'
> irb(main):092:1> end
> "Rocky Raccoon"
> "As My Guitar Gently Weeps"
> "She Came In Through The Bathroom Window"
> "no songs!"
> => nil
> irb(main):093:0>
>
> The presence of 'no songs!' is not what I'd expected.
>>
>> Yours is different from Hal's, because in Hal's the iteration through
>> the song list is sort of a side-effect of the testing of the
>> condition, whereas you're just doing a standard chaining thing.  And
>> you do create a temporary array :-)  (the return value of #reject)
>
> This makes sense in isolation, however given the above result I'm 
> curious as to what is actually going on.  As to the temporary 
> variable, I am more tired of having "foo = []" all over my code.
>
>> You could perhaps make it easier to read with a different formatting:
>>
>>   ary.reject {|e| e.some_predicate? } .each do |f|
>>     f.do_something
>>   end
>>
>> or something.  (Definitely lose the one-space indenting.)  If your
>> friend still has trouble with it, buy him the Pickaxe :-)
>
> I agree the {} block looks more like something that yields a value 
> than do..end.  I think I had it that way at first but the line spilled 
> over past column 80 so I changed it to a do...end, which I use when a 
> block spans multiple lines.  Is that a common idiom or am I unique in 
> this regard?
>
> --
> ~akk
> http://therealadam.com
>
>
>
--
(-, /\ \/ / /\/