From: Adam Shelly <adam.shelly@...>
Date: 2008-08-29T02:36:18+09:00
Subject: Re: Writing a method to handle a code block?

On 8/28/08, Brian Ross <p.brian.ross@gmail.com> wrote:
> From Beginning Ruby:
>
> def each_vowel(&code_block)
>  %w{a e i o u}.each { |vowel| code_block.call(vowel) }
> end
> each_vowel { |vowel| puts vowel }
>
> I am trying to figure out how that works but I'm still having a bit of
> trouble. Could someone break it down bit by bit to show what it's doing?
>
> def each_vowel(&code_block)
>
> It defines a method that takes a code block (is the & necessary?).

Yes the & is necessary, otherwise when you try to call this method
with `each_vowel {|v|puts v}` you will get an argument error, since
the method will expect a normal object, not a block.

> What does it mean to have a method that takes a code block?

At first approximation you can think of a code block as an anonymous method.
You are writing a method that can take another method as an argument.

>
>  %w{a e i o u}.each { |vowel| code_block.call(vowel) }
>
> Then it takes an array of vowels, which call the each method to pass each
> one into the following block through |vowel| as a block argument. The block
> arguments are then called by the variable code_block (I don't understand
> this).

code_block.call(vowel) is calling the method stored in the code_block
variable, and
passing it the argument called vowel.  When you call each_vowel like this:

each_vowel {|vowel| puts vowel}

The block with 'puts' is stored as code_block, and run 5 times, once
for each letter in the array:
 %w{a e i o u}.each { |vowel| code_block.call(vowel) }

So the end result is that each vowel is printed out.


HTH,
-Adam