Get an array of the values of a key from an array of hashes?

Question

For an array like this:

 a = [{a:'a',b:'3'},{a:'b',b:'2'},{a:'c',b:'1'}]

I would like to return an array containing values for :a keys, so:

 ['a', 'b', 'c']

That can be done using:

 a.map{|x|x[:a]}

I wonder if there is a native method in Rails or Ruby to do it like this?

 a.something :a

Answer 1

You can do it yourself:

class Array
  def get_values(key)
    self.map{|x| x[key]}
  end
end

Then you can do this:

a.get_values :a
#=> ["a", "b", "c"]

Answer 2

More than you need in this case, but from How to merge array of hashes to get hash of arrays of values you can get them all at once:

merged = a.inject{ |h1,h2| h1.merge(h2){ |_,v1,v2| [*v1,*v2] } }
p merged[:a] #=> ["a", "b", "c"]
p merged[:b] #=> ["3", "2", "1"]

Also, if you use something like Struct or OpenStruct for your values instead of hashes—or any object that allows you to get the "a" values as a method that does not require parameters—you can use the Symbol#to_proc convenience for your map:

AB = Struct.new(:a,:b)
all = [ AB.new('a','3'), AB.new('b','2'), AB.new('c','1') ]
#=> [#<AB a="a", b="3">, #<AB a="b", b="2">, #<AB a="c", b="1">]

all.map(&:a) #=> ["a", "b", "c"]
all.map(&:b) #=> ["3", "2", "1"]