How do I calculate the average values in objects located in an array?

Question

Let's say I have an array like this:

[
  {
    "player_id"         => 1,
    "number_of_matches" => 2,
    "goals"             => 5
  },
  {
    "player_id"         => 2,
    "number_of_matches" => 4,
    "goals"             => 10
  }
]

I want to have the average goals per match among all the players, not the average for each individual player, but the total average.

I have in mind doing it with .each and storing each of the individual averages, and at the end add them all and divide by the number of players I have. However, I am looking for a Ruby/ one-liner way of doing this.

Answer 1

As requested, a one-liner:

avg = xs.map { |x| x["goals"].to_f / x["number_of_matches"] }.reduce(:+) / xs.size

A more readable snippet:

goals, matches = xs.map { |x| [x["goals"], x["number_of_matches"]] }.transpose 
avg = goals.reduce(:+).to_f / matches.reduce(:+) if goals

Answer 2

A slight modification to tokland's answer.

items.map{|e| e.values_at("goals", "number_of_matches")}.transpose.map{|e| e.inject(:+)}.instance_eval{|goals, matches| goals.to_f/matches}