From: Dante Regis <dante.regis@...>
Date: 2007-10-11T11:51:43+09:00
Subject: Re: |how do you know when to put a variable between pipes?|

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Another way to explain, complementing our friend Morton, is that when you
use a block, you are kind of inserting code inside another function. Let me
give you an example to help you understand.

Let's take the "each" method of an array. You use this method when you want
to do something with every item inside the array.

You could call something like:
my_string = ""
my_array.each do |foo|
  puts foo
  my_string += " AND #{foo}"
end

Now, why did we place that |foo| on the block? Imagine that the "each"
method on the array is implemented like this:

def each
   i = 0
   while i <= self.size
      current_item = self[i]
      yield(current_item)
   end
   self
end

Now notice two things: First, the "each" method itself has no parameters,
though it could have. They would have nothing to do with the pipes, though.
Second, look at the yield line. what yield do is to call the block that you
gave to this function. Since the yield is inside the while statement, it
will run as many times as there are items on your array. And, as i said it
is just like putting your piece of code inside the "each" (or any other)
method. It would be like you've written

 def my_each
   my_string = ""
   i = 0
   while i <= self.size
      current_item = self[i]
      # ----
      puts current_item
      my_string += " AND #{foo}"
      # -----
   end
   self
end

Now, the actual values that the block will take will clearly depend on the
function that you are using. SInce yield can pass as many parameter as it
wants (you could have something like yield a, b, c, d, e, ...) you should
see the documentation of the function to know which values are passed to the
block.

It's obviously not a technical explanation, and some things are
intentionally wrong (the each method for array is implemented in C, for
instance), but maybe it helps you understand.

Hope this helps

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