How do I map an array of hashes?

Question

I have an array of hashes:

arr = [ {:a => 1, :b => 2}, {:a => 3, :b => 4} ]

What I want to achieve is:

arr.map{|x| x[:a]}.reduce(:+)

but I think it's a bit ugly, or at least not that elegant as:

arr.map(&:a).reduce(:+)

The later one is wrong because there is no method called a in the hashes.

Are there any better ways to write map{|x| x[:a]}?

Answer 1

You could make actual Objects, possibly with a Struct:

MyClass = Struct.new :a, :b
arr = [MyClass.new(1, 2), MyClass.new(3, 4)]
arr.map(&:a).reduce(:+)  #=> 4

Or for more flexibility, an OpenStruct:

require 'ostruct'
arr = [OpenStruct.new(a: 1, b: 2), OpenStruct.new(a: 3, b: 4)]
arr.map(&:a).reduce(:+)  #=> 4

Of course either of these can be constructed from existing hashes:

arr = [{ :a => 1, :b => 2 }, { :a => 3, :b => 4 }]

ss = arr.map { |h| h.values_at :a, :b }.map { |attrs| MyClass.new(*attrs) }
ss.map(&:a).reduce(:+)  #=> 4

oss = arr.map { |attrs| OpenStruct.new attrs }
oss.map(&:a).reduce(:+)  #=> 4

Or, for a more creative, functional approach:

def hash_accessor attr; ->(hash) { hash[attr] }; end
arr = [{ :a => 1, :b => 2 }, { :a => 3, :b => 4 }]
arr.map(&hash_accessor(:a)).reduce(:+)  #=> 4