What's the best way in Ruby to average sets in an array?

Question

Given the following simple data set, what is the best way to average the values for the sets 0 25 53 and 80.

  [["0", "148.5"],
   ["0", "146.5"],
   ["0", "148.6"],
   ["0", "202.3"],
   ["25", "145.7"],
   ["25", "145.5"],
   ["25", "147.4"],
   ["25", "147.3"],
   ["53", "150.4"],
   ["53", "147.6"],
   ["53", "147.8"],
   ["53", "215.4"],
   ["80", "150.4"],
   ["80", "149.4"],
   ["80", "148.0"],
   ["80", "149.9"]]

Answer 1

It's simple enough with inject. I often implement a general group_by method in projects to help with stuff like this.

If data is large and performance matters consider using a numeric library or database if appropriate.

data = [ ... ]

groups = data.inject({}) do |hash, pair| 
  hash[pair.first] ||= []
  hash[pair.first] << pair.last.to_f
  hash
end

groups.inject({}) do |hash, pair| 
  hash[pair.first] = pair.last.inject(0,&:+) / pair.last.size
  hash
end

Answer 2

Using inject with a hash will yield poor performance (you're re-assigning the memo var at every iteration). If you're on 1.9, Enumerable implements the method group_by, which can be used to make the code a little more obvious:

result = array.map{ |row| [row.first.to_i, row.last.to_f] }.group_by(&:first)
result.each_pair do |key, values|
  result[key] = values.average
end

Array#average is easily implemented as

class Array
  def average
    inject(0.0) { |sum, e| sum + e } / length
  end
end

The fact that your data is strings is quite inconvenient, I recommend avoiding that whenever possible.