From: Brad <coish@...>
Date: 2003-12-14T07:26:58+09:00
Subject: Re: Simple issue giving problems

Jim Weirich wrote:

> Brad wrote:
> 
> > What I was trying to say was...
> > 
> > I was wondering if there were some way to allow manipulations of
> > function/methods parameters such that the calling processes variable
> > values would be modified.  
> > 
> > As I understand it now this is not possible, though being new to Ruby I 
> > thought it couldn't hurt to ask.
> 
> No, it doesn't hurt to ask.
> 
> There is a way of modifying the variable bindings in the calling method, 
> but it is not widely used.  You need to create a "reference" to the 
> variable and pass that into the swap function.  I use a "helper" 
> function "ref" to create the reference ...
> 
> Here's an example ...
> 
>    require 'reference'
> 
>    def swap(a,b)
>      a.value, b.value = b.value, a.value
>    end
> 
>    x = 10
>    y = 11
> 
>    puts "BEFORE: x = #{x}, y = #{y}"
>    swap(ref{:x}, ref{:y})
>    puts "AFTER:  x = #{x}, y = #{y}"
> 
> Note that those are curly braces, not parenthesis after "ref".  This 
> will print ...
> 
>    BEFORE: x = 10, y = 11
>    AFTER:  x = 11, y = 10
> 
> Here's the reference.rb file that you will need ...
> 
>    class Reference
>      def initialize(&block)
>        sym = block.call
>        fail "Block must yield symbol" unless Symbol === sym
>        @getter = eval "proc { #{sym} }", block
>        @setter = eval "proc { |v| #{sym} = v }", block
>      end
>      def value
>        @getter.call
>      end
>      def value=(v)
>        @setter.call(v)
>      end
>    end
> 
>    def ref(&block)
>      Reference.new(&block)
>    end
> 
> 

Jim:

Thanks for the reply, it's appreciated.  I think this may be what I'm
looking for.  I'll give it a try and get back to you with the results.


Thanks,
Brad