Why does the function not work when using splat in the parameter

Question

When you remove the * splat from the parameter the function works fine. However when it's in there the function does not work. Why? See my repl (https://repl.it/LuY8/4)

class Hash

  def keys_of(*arguments)

    new_array = []

    self.each do |key, value|

      #puts arguments

      #puts key

      if value == arguments

        new_array << key 

      end

    end

    new_array

  end

end



animals = {"sugar glider"=>"Australia","aye-aye"=> "Madagascar","red-footed tortoise"=>"Panama","kangaroo"=> "Australia","tomato frog"=>"Madagascar","koala"=>"Australia"}



animals.keys_of('Madagascar')

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Answer 1

That's a varargs method signature, that is arguments will always be an array, even for singular values. That means you're getting ['Madagascar'] as your arguments and since your keys aren't arrays of a single string, your match fails.

What you probably want is to invert the whole routine and make it more Ruby-like by doing this:

def keys_of(*arguments)
  each_with_object([ ]) do |(key, value), a|
    a << key if (arguments.include?(value))
  end
end

Problem solved. Ruby has a very rich and featureful core library and Enumerable is the real jewel. Familiarize yourself with what it can do before writing your own work-alike methods.

Why does the function not work when using splat in the parameter

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