From: Walton Hoops <walton@...>
Date: 2009-12-15T11:07:38+09:00
Subject: Re: Newbie Question: Blocks and Parameters

> -----Original Message-----
> From: derekwrobel@gmail.com [mailto:derekwrobel@gmail.com]
> Please have patience with my possibly basic basic question.  I am
> learning Ruby and studying blocks/closures right now and confused
> without being able to find an answer.  I was studying and first came
> across this example
> 
> [1,2,3].each {|x| puts "This is #{x}. "}
> 
> 
> and this makes sense to me I believe.  for each object (which is
> integers 1, 2, 3), it runs the {} block code, passing in the integer
> objects in as variable 'x', then puts out the string including the 'x'
> variable.
> 
> Now what gets me confused is once I get to these following examples:
> 
> class MyClass
>   def command1()
>     yield(Time.now)
>   end
> end
> 
> m = MyClass.new
> m.command1() {|x| puts "Current time is #{x}"}
> 
> In the above example, how does 'x' get the Time.now value?  In the
> first
> code I have above, 'x' becomes the actual integer object (1, 2, and 3).
> So in this case, how does the 'x' variable in this block code get the
> Time.now value when executed through yield?

You can pass any function a block, whether it will use it, or not. In
order to execute the block passed to a function, that function must 
call yield.  The arguments to yield get passed in as arguments to the
block.

Going back to your first example.  A possible implementation of .each

class Array
  def each
    x=0
    while x<self.size
      yield(self[x])
      x+=1
    end
  end
end

Thus, in this example calling each calls yield passing in one at a time
each member of the array.  Thus your block gets excuted once for each
member of the array.  Does that help? I'm not sure how to explain it
better.