From: Robert Klemme <shortcutter@...>
Date: 2007-10-20T18:30:04+09:00
Subject: Re: the difference between for-loop and each

On 19.10.2007 21:19, Andreas S wrote:
> Hopefully (I'm sure) somebody can shed a light on this. This caught me by surprise
> 
> test.rb:
> TEST = []
> def procs &block
>   TEST << block
> end
> 
> #for n in [1,2,3] do
> [1,2,3].each do |n|
>   procs do
>     puts "#{n}"
>   end
> end
> 
> TEST.each do |t|
>   puts t
>   t.call
> end
> 
> With for loop
>>> ruby test.rb
> #
> 3
> #
> 3
> #
> 3
> 
> With each
>>> ruby test.rb
> #
> 1
> #
> 2
> #
> 3
> 
> I thought for-loop behaves the same as each. Apparently not. Why is this and is this a good thing?

The reason for the behavior you are seeing is different scoping.  "for" 
does not open a new scope while block parameters sit in a different 
scope.  Apart from that "for" is a keyword (i.e. part of the language) 
while "each" is just a method like other methods.

irb(main):014:0> def f1(x) x.each  {|y| p y}; p y end
=> nil
irb(main):015:0> def f2(x) for y in x; p y; end; p y end
=> nil
irb(main):016:0> f1 %w{foo bar baz}
"foo"
"bar"
"baz"
NameError: undefined local variable or method `y' for main:Object
         from (irb):14:in `f1'
         from (irb):16
         from :0
irb(main):017:0> f2 %w{foo bar baz}
"foo"
"bar"
"baz"
"baz"
=> nil
irb(main):018:0>

I'd say it's a good thing because you get the choice - if you need the 
iteration variable after the block then use "for" - otherwise use "each".

Kind regards

	robert