From: RichardOnRails <RichardDummyMailbox58407@...>
Date: 2011-01-24T03:10:34+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 Jan 23, 12:26 am, Jeremy Bopp <jer...@bopp.net> wrote:
> 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

Hi Jeremy,

Thanks for that lucid explanation of where my error lay which in turn
made the + method inappropriate.  The app I'm working do have class
and relevant methods defined,  so using the scheme I playing with is
fine in a class context.

Again, thanks,
Richard