From: Paul Harrington <xeno@...>
Date: 2010-09-09T05:22:01+09:00
Subject: Re: Problem with Hash of Arrays

Fearless Fool wrote:
> This is a long-inactive thread, but it glosses over an important gotcha 
> that nobody has mentioned.
> 
> As mentioned above, the sexy ruby-esque way to make a hash of arrays is 
> to use a block initializer for Hash.new:
> 
>>> h = Hash.new{|hash, key| hash[key] = []}
> => {}
> 
> This allows you to push items onto any slot of the hash without first 
> checking to see if it's nil:
> 
>>> h[:first] << "yum!"
> => ["yum!"]
>>> h[:first] << "tastes like chicken"
> => ["yum!", "tastes like chicken"]
> 
> But here's the gotcha: If you later test for a non-empty slot with 
> h[<key>], h will automatically create an empty array whether you 
> intended to or not.  Consider:
> 
> process(h[:second]) if h[:second]
> 
> Even though we never pushed anything onto h[:second], process() WILL get 
> called (and the hash will grow by one element), since the test 
> h[:second] automagically creates an empty array.  This may not be a 
> problem if you use h entirely within your own code, but could be 
> surprising behavior -- to you or to some other user -- and lead to 
> elusive bugs.  (As I discovered...)
> 
> MORAL: A better idiom for pushing items onto a hash of arrays is:
> 
>>> h = Hash.new
>>> (h[:first] ||= [])  << "yum!"
> 
> ... since this avoids any unexpected behavior with the hash later on.
> 
> - ff

This here is why you learn the core library.

irb(main):025:0> h = Hash.new {|h, k| h[k] = []}
=> {}
irb(main):026:0> h[:rocks].push "cliff"
=> ["cliff"]
irb(main):027:0> h[:rocks]
=> ["cliff"]
irb(main):028:0> h.has_key? :face
=> false
-- 
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