From: Joel VanderWerf <vjoel@...>
Date: 2008-12-24T02:26:17+09:00
Subject: Re: Loading up a hash with variables if they are defined

Patrick Doyle wrote:
> Is there a cleaner, more Rubyesque way to do this?
> 
>   # Return a hash of the navigation parameters
>   def nav_params
>     params = {}
> 
>     params[:prefix]      = @prefix      if @prefix
>     params[:category_id] = @category_id if @category_id
>     params[:page]        = @page        if @page
> 
>     params
>   end
> 
> I want to load a parameters hash with the contents of some instance
> variables, but only if they are defined.

Watch out: "only if they are defined" is not the same as "only if they 
are not nil or false". If you want the former, then this might be what 
you are looking for:

class C
   NAV_VARS = %w{ @prefix @category_id @page }
   NAV_VAR_KEYS = NAV_VARS.inject({}) {|h,v| h[v] = v.delete("@").to_sym; h}

   def nav_params
     (instance_variables & NAV_VARS).inject({}) do |params,var|
       params[NAV_VAR_KEYS[var]] = instance_variable_get(var)
       params
     end
   end
end

c = C.new
c.instance_eval do
   @prefix = "foo"
   @page = false
end

p c.nav_params # ==> {:prefix=>"foo", :page=>false}

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407