Forum: Ruby Loading up a hash with variables if they are defined

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Patrick D. (Guest)
on 2008-12-23 19:00
(Received via mailing list)
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.

--wpd
James C. (Guest)
on 2008-12-23 19:07
(Received via mailing list)
>  end
>
> I want to load a parameters hash with the contents of some instance
> variables, but only if they are defined.



params = [:prefix, :category_id, :page].inject({}) do |hash, name|
  var = instance_variable_get("@#{name}")
  hash[name] = var if var
  hash
end
Jesús Gabriel y Galán (Guest)
on 2008-12-23 19:14
(Received via mailing list)
On Tue, Dec 23, 2008 at 5:51 PM, Patrick D. 
<removed_email_address@domain.invalid>
wrote:
>    params
>  end
>
> I want to load a parameters hash with the contents of some instance
> variables, but only if they are defined.
>


irb(main):005:0> {:prefix => @prefix, :category_id => @category, :page
=> @page}.reject {|k,v| v.nil?}
=> {:prefix=>"asdasdf", :category_id=>"cat"}

Jesus.
Patrick D. (Guest)
on 2008-12-23 19:23
(Received via mailing list)
On Tue, Dec 23, 2008 at 11:57 AM, James C.
<removed_email_address@domain.invalid>wrote:

> >  end
> end
>
Thanks... that's certainly along the lines of what I was searching for.
I'll go read about #instance_variable_get now.

--wpd
Joel VanderWerf (Guest)
on 2008-12-23 19:35
(Received via mailing list)
Patrick D. wrote:
>     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}
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