From: Rob Biedenharn <Rob@...>
Date: 2007-01-21T03:36:58+09:00
Subject: Re: Proper way to either create or append to an array?


On Jan 20, 2007, at 1:15 PM, Phrogz wrote:

> Robert Klemme wrote:
>> Since you're using a Hash the block constructor is the most elegant
>> solution IMHO:
>>
>> def initialize
>>    @foo = Hash.new {|h,k| h[k]=[]}
>>    ...
>> end
>
> To be clear for those unfamiliar with this: any time you ask for the
> value of a key that doesn't exist, the block will be called (which
> creates a new empty array). This means you can never see this
> particular Hash has a key, since:
>   if @foo[ :bar ]
> will always succeed, and create a (possibly unwanted) array  
> instance in
> the process.

That's only because you're not checking for the presence of a key  
correctly:

 >> myhash = Hash.new { |h,k| h[k] = Array.new }
=> {}
 >> if myhash[:oops]
 >>   puts "nothing"
 >>   end
nothing
=> nil
 >> myhash
=> {:oops=>[]}
 >> if myhash.has_key? :missing
 >>   puts ":missing contains #{myhash[:missing] * ', '}"
 >>   end
=> nil
 >> myhash
=> {:oops=>[]}

If you know that your code isn't using the block initialization, then  
the first form might be OK.  However, the #has_key? will always do  
what you expect.  This is particularly true if the value can be nil  
or false.

-Rob


Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com