From: Jeff Nyman <jeffnyman_noemail@..._gmail.com>
Date: 2006-10-04T02:15:09+09:00
Subject: Re: Using Each to Iterate

[Sending this from another account, so I'm sorry if this double-posts.]

With all the responses I got here, I figured it out. Thanks to all of you.
I'll put the relevant source here and what I did, just in case this helps
others.

===============

First, a bit of explanation. I have a Step class. I also have a StepList
class. (This is similar to the Song and SongList class in the Pickaxe book.)

The idea is that when a new step is created, I do so like this:

$stepList.append(Step.new(firstStep, thisStep, thisFilter))

So here are the classes:

<code>
class Step
  attr_reader :point1, :point2, :filter

  def initialize(point1, point2, filter=nil)
    @point1 = point1
    @point2 = point2
    @filter = filter
  end

end

class StepList
  def initialize
    @guides = Array.new
  end

  def append(thisGuide)
    @guides.push(thisGuide)
    self
  end

  def filter(thisGuide)
    @guides.find { |guide| thisGuide == guide.filter }
  end

  def [](index)
    @guides[index] if index.kind_of?(Integer)
  end

  def each
    @guides.find { |guide| guide.filter }
  end
end
</code>

Then I have $stepList referencing a StepList object. When I have added all
the steps and just do a print of $stepList, I get this:

#<StepList:0x2f37954 @guides=[#<Step:0x2a9ad10 @filter="first after
202G_OrdAdd,203G_OrdUp
dateFirst", @point2="203G_OrdUpdate", @point1="202G_OrdAdd">,
#<Step:0x2a99e88 @filter="la
st,203G_OrdUpdateLast", @point2="203G_OrdUpdate", @point1="202G_OrdAdd">,
#<Step:0x2a96a94
 @filter="last,203G_OrdUpdateLast", @point2="203G_OrdUpdate",
@point1="203G_OrdUpdate">]>

So what I wanted to do is filter through all the Step objects that are in
the StepList and then check if they have a filter set.

So here was the logic I had:

$stepList.each { |thisFilter|
  puts thisFilter
}

What my hope was is that the "each" method of my StepList class would return
me the filters. And it did -- if I changed my method like this:

def each
  @guides.find { |guide| puts guide.filter }
end

(or if I used "yield" as some had suggested). With the puts or yield in
place, I saw this when the code for the $stepList.each is run:

first after 202G_OrdAdd,203G_OrdUpdateFirst
last,203G_OrdUpdateLast
last,203G_OrdUpdateLast

Those are definitely the filters. So what I was hoping was that each of
those would, in turn, get sent to thisFilter in my each loop. That appeared
to not happen. If I changed the 'each' method to a 'selection' method
(Paul's idea), I got this if I had yield in place in the method:

./classes.rb:118:in `selection': no block given (LocalJumpError)

However, if I did not have yield in place, I got this:

undefined method `each' for #<Step:0x2a9a98c>

That seemed to make sense since I needed an 'each' method for my Step class.
So I added this 'each' method to the Step class:

def each
  yield @filter
end

That seems to do the trick perfectly.

- Jeff