Apply a method to an array and return array value as a key

Question

I would like to know if ruby has a builtin method to do the following.

from this array:

array = [:foo, :bar]

and this method:

def content_for_key key
  return :baz if key == :foo
  return :qux if key == :bar
end

and this call:

array.some_built_in_ruby_method(&:content_for_key)

we get:

{
  :foo => :baz,
  :bar => :qux,
}

Can you tell how the result is constructed? content_for_key returns nil for anything other than :bar, so what is the logic to build the result? Any time content_for_key returns nil, we map that to :baz? — Jörg W Mittag
– Jörg W Mittag, Commented Jun 14, 2018 at 7:57

Stefan · Accepted Answer · 2018-06-14 08:34:46Z

4

You could use map to convert each element to a [key, value] pair:

array.map { |key| [key, content_for_key(key)] }
#=> [[:foo, :baz], [:bar, :qux]]

followed by to_h to transform the nested array into a hash:

array.map { |key| [key, content_for_key(key)] }.to_h
#=> {:foo=>:baz, :bar=>:qux}

Or you could use each_with_object to populate a hash while traversing the array:

array.each_with_object({}) { |key, hash| hash[key] = content_for_key(key) }
#=> {:foo=>:baz, :bar=>:qux}

edited Jun 14, 2018 at 8:34

answered Jun 14, 2018 at 8:00

Stefan

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4 Comments

Daniel Costa Over a year ago

My question is: "Does ruby offer a Array method taking a symbol &:my_func (like mao) and returning an hash with one key per array item and the value of this item would be my_func(item)

Stefan Over a year ago

@DanielCosta maybe Ruby does have such method and I just prefer posing long-winded solutions instead. But that's rather unlikely ;-)

Daniel Costa Over a year ago

I actually implemented the .to_h earlier today before posting, was just wondering. Thanks for the answer though

Sergio Tulentsev Over a year ago

@DanielCosta: no, array doesn't have such method. Nor does anything else. It's just how &:my_func works. It generates a block which, when passed elem, runs elem.my_func. That's it.

engineersmnky · Accepted Answer · 2018-06-14 13:29:57Z

1

You could go with something like this. Although it lacks readability in comparison to the answer you already have

array = [:foo, :bar, :mnky]
def content_for_key key
  return :baz if key == :foo
  return :qux if key == :bar
end
array.zip(array.map(&method(:content_for_key))).to_h
#=> {:foo=>:baz, :bar=>:qux, :mnky => nil}

# or 

[array,array.map(&method(:content_for_key))].transpose.to_h
#=> {:foo=>:baz, :bar=>:qux, :mnky=>nil}

edited Jun 14, 2018 at 13:29

answered Jun 14, 2018 at 13:03

engineersmnky

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