From: Jeremy Bopp <jeremy@...>
Date: 2011-01-23T14:26:24+09:00
Subject: Re: Easiest way in Ruby to express "given this, set target to this if nil or undefined, else increment by this"

On 01/22/2011 10:50 PM, RichardOnRails wrote:
> The following works,  but I'd prefer not to have to class values:
> 
> class Z
>     def self.show_z
> 	@@zz ||= 0; @@zz += 1
> 	puts @@zz
>     end
> end
> Z.show_z	# => 1
> Z.show_z	# => 2

Replace the @@'s with @'a so that you use a class instance variable
instead.  In this case it works effectively the same while avoiding the
nasty class variables.

> I tried this:
> 
> $h = Hash.new(:y)
> def show_y
>     $h[:y] ||= 0; $h[:y] += 1 # => Error msg # Line 18
>     puts $h[:y]
> end
> show_y
> show_y
> 
> but it doesn't pass muster with Ruby 1.8.6.  I get a complaint about
> the first line in show_y, which makes no sense IMHO:
> 
> TestOrEqual_operator.rb:18:in `show_y': undefined method `+'
> for :y:Symbol (NoMethodError)
> 	from TestOrEqual_operator.rb:21

The error is ultimately the result of the way you initialized your Hash
instance.  By passing :y to the new method, you arranged for any
uninitialized key referenced within the hash to have the symbol :y as a
value.  Therefore,

$h[:y] ||= 0

doesn't do anything because $h[:y] has a value of :y by default.  The
next statement then attempts to increment the value of $h[:y] by 1 and
fails because that value is :y and the + method is not defined for
Symbols.  If what I'm saying isn't exactly clear, try this:

h = Hash.new(:howdy)
puts h['Say hello like a Texan!']
puts h['Now say hello like a John Wayne!']

You could avoid this particular problem by simply setting $h to {}
rather than calling Hash.new(:y).  But why is a hash necessary for this
at all???  You could just as easily do this:

def show_y
  $y ||= 0
  $y += 1
  puts $y
end

This still leaves you using a global, but it's much simpler.  More
importantly, it works. ;-)

> Also, I'd be able to use a succinct version of this without resorting
> to globals.

Without knowing more about the problem you're trying to solve, it's hard
to provide a better response.  Apparently, you want to call a method
that automatically initializes some state when needed and then modify
and preserve that state between subsequent calls to that method.  That
state, be it a number as in your examples here or something else, has to
be stored somewhere.

I'm not a fan of globals in general, so I would choose to keep the state
in a class as you did in your first example.

-Jeremy