Ruby Beginner Class Question

Question

I'm trying to learn ruby more in depth before I move on to rails dev, but I'm having some issues learning classes. I can't seem to understand why the following doesn't work.

#point.rb
class Point
  attr_accessor :x, :y

  def initialize(p = [0,0])
   @x = p[0]
   @y = p[1]
  end
end

#shape.rb
require_relative 'point.rb'

class Shape

  attr_accessor :points

  def initialize *the_points
    for p in the_points
      @points.append Point.new(p)
    end
  end

end

s = Shape.new([3,2])

puts s.points

When I call the function I get a no method error for NilClass, which I'm assuming is referring to @point.append.

score 5 · Accepted Answer · edited May 02 '11 at 17:49

5

First, try this:

def initialize *the_points
  @points = []
  for p in the_points
    @points << Point.new(p)
  end
end

You get NilClass error because @points instance variable is Nil, and NilClass, which does not have append() method.

edited May 02 '11 at 17:49

Phrogz

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answered May 02 '11 at 17:22

rmk

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Now I'm getting a different error: `shape.rb:11:in block in initialize: undefined method 'append' for []:Array (NoMethodError)` Which is really confusing to me, since I'd think that array would be fine with append. I'm on ruby 1.9.2 if that matters. – tshauck May 02 '11 at 17:31
1

@tshauck - you can use `push` or `<<` instead, there is no `append` method on Array. – McStretch May 02 '11 at 17:33
Hmm... This is a little surprising, but I don't see Array#append in Ruby 1.9.2. You should probably use << (e.g., @points << Point.new(p)) – rmk May 02 '11 at 17:37
muchas gracias @rmk and @McStretch – tshauck May 02 '11 at 17:37

score 1 · Answer 2 · answered May 02 '11 at 17:51

1

Better than creating an array and populating it in a loop would be to initialize it like so:

class Shape
  attr_accessor :points

  def initialize *the_points
    @points = the_points.map{ |p| Point.new(p) }
  end
end

answered May 02 '11 at 17:51

Phrogz

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Point-free style FTW: `the_points.map(&Point.method(:new))`. Actually, I had this idea that classes are procedures, namely they are procedures that produce instances. And just like any object which is a procedure in Ruby, they should implement `to_proc`, so that you could write `the_points.map(&Point)`. The monkey patch for that is simple: `class Class; def to_proc; proc(&method(:new)) end end`. – Jörg W Mittag May 03 '11 at 03:35
@Jörg I wish you got rep for comment upvotes, because that's some crazy good thinking you're sharing here. :) – Phrogz May 03 '11 at 03:51
I think I posted that somewhere. At least I just found it in my StackOverflow snippets directory, but maybe it's for an answer I meant to give but never posted. – Jörg W Mittag May 03 '11 at 03:57

score 1 · Answer 3 · edited May 23 '17 at 12:03

1

If you had warnings on (ruby -w or $VERBOSE = true), it'd warn you that @points didn't exist.

See some other debugging tips in How do I debug Ruby scripts?

edited May 23 '17 at 12:03

Community

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answered May 02 '11 at 23:42

Andrew Grimm

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score 0 · Answer 4 · answered May 02 '11 at 17:23

0

You need to initialize @points to be a new array. It starts off as nil.

  def initialize *the_points
    @points = [];
    for p in the_points
      @points.append Point.new(p)
    end
  end

answered May 02 '11 at 17:23

Vadim

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Ruby Beginner Class Question

4 Answers4