From: Brian Candler <b.candler@...>
Date: 2008-11-18T19:03:22+09:00
Subject: Re: function to select only certain key/value pairs from has

Aryk Grosz wrote:
> I want to do something like this
> 
> hash.from_keys(:a,:b) => {:a=> 1, :b=> 2}

In ruby1.9, Hash#select returns another Hash. But you'd still be 
iterating the 'wrong way' (that is, iterating through the hash and doing 
a linear search through the keys)

Personally I'd go with:

class Hash
  def from_keys(*keys)
    keys.inject({}) { |h,k| h[k] = self[k] if has_key?(k); h }
  end
end

hash = {:a => 1, :b => 2, :c => 3}
p hash.from_keys(:a, :b)

With ruby19 you can do:

    keys.each_with_object({}) { |k,h| h[k] = self[k] if has_key?(k) }

which is more keystrokes but maybe the teeniest bit more efficient. But 
I hate each_with_object on the principle that its arguments are the 
opposite way round to inject :-(
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