From: Trans <transfire@...>
Date: 2007-07-13T03:41:45+09:00
Subject: Re: Returning part of a hash



On Jul 11, 4:30 pm, barjunk <barj...@attglobal.net> wrote:
> I have hash that has about 20 keys.  I'd like to create a new variable
> with just three of those keys.  Example:
>
> hash = { "key1" => "value1",
>                "key2" => "value2",
>                ...
>                "key20" => "value20" }
>
> And a function like:
> newhash = hash.slice("key2","key5","key7")
>
> Which creates:
>
> newhash = { "key2" => "value1",
>                "key5" => "value5",
>                "key7" => "value7" }
>
> hash.select {|key, value| key == "key1" }
>
> I could do the above multiple times, but this returns an array not the
> hash pair.
>
> Thanks for any help.

  class Hash

    # Hash intersection. Two hashes intersect
    # when their pairs are equal.
    #
    #   {:a=>1,:b=>2} & {:a=>1,:c=>3}  #=> {:a=>1}
    #
    # A hash can also be intersected with an array
    # to intersect by keys only.
    #
    #   {:a=>1,:b=>2} & [:a,:c]  #=> {:a=>1}
    #
    # The later form is similar to #pairs_at. This differs only
    # in that #pairs_at will return a nil value for a key
    # not in the hash, but #& will not.

    def &(other)
      case other
      when Array
        k = (keys & other)
        Hash[*(k.zip(values_at(*k)).flatten)]
      else
        Hash.new[*(to_a & other.to_a).flatten]
      end
    end

  end

T.

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