Question

I have a set of 2 or more objects that I’d like to order. I had been doing it like this:

card.max_by{|strength| strength.score

Where score was an integer score I had computed given some arbitrary rules. I knew this would be something I would refactor, so now I am doing so. And the “clean” way to give a score to a hand is to give it an array of values like

foo.score = [9,3,nil,4]

And compare it to another hand which might have an array like

bar.score = [5,10,12,12]

And foo <=> bar would tell me that foo is the greater array and so it should be returned by max_by. The problem is that max_by apparently won’t make comparisons on arrays. Is there another way I can do this to sort by the array value?

Answer 1

If it’s just array-based ordering that you want (you really do want the spaceship operator) and you want to find the ‘biggest’ by sorting, then:

Hand = Struct.new(:score)

hands = [
 Hand.new([9,3,0,4]),
 Hand.new([8,8,8,8]),
 Hand.new([5,10,12,12]),
 Hand.new([1,99,99,99])
]

biggest = hands.sort_by(&:score).last
p biggest
#=> #<struct Hand score=[9, 3, 0, 4]>

If you really only need to find the largest hand, however, the following will be more efficient than ordering the entire array:

biggest = hands.inject do |max,hand|
  if (max.score <=> hand.score) == -1
    hand
  else
    max
  end
end
p biggest
#=> #<struct Hand score=[9, 3, 0, 4]>

Edit: Reading your comment, if you really need multiple values that match, I would do this:

Hand = Struct.new(:name,:score) do 
  MAX_SCORE_PART = 13 # 13 ranks in a suit
  def numeric_score
    value = 0
    score.each_with_index do |part,i|
      value += part.to_i * MAX_SCORE_PART**(score.length-i-1)
    end
    value
  end
end

hands = [
  Hand.new('Bob', [9,3,nil,4] ),
  Hand.new('Jim', [8,8,8,8]   ),
  Hand.new('Foo', [5,10,12,12]),
  Hand.new('Sam', [1,13,13,13]),
  Hand.new('Zak', [9,3,0,4]   ),
]

require 'pp'
by_score = hands.group_by(&:numeric_score)
pp by_score
#=> {20284=>
#=>   [#<struct Hand name="Bob", score=[9, 3, nil, 4]>,
#=>    #<struct Hand name="Zak", score=[9, 3, 0, 4]>],
#=>  19040=>[#<struct Hand name="Jim", score=[8, 8, 8, 8]>],
#=>  12843=>[#<struct Hand name="Foo", score=[5, 10, 12, 12]>],
#=>  4576=>[#<struct Hand name="Sam", score=[1, 13, 13, 13]>]}

pp by_score[by_score.keys.max]
#=> [#<struct Hand name="Bob", score=[9, 3, nil, 4]>,
#=>  #<struct Hand name="Zak", score=[9, 3, 0, 4]>]

For an inject-based implementation:

def numeric_score
  score.enum_for(:inject,0).with_index do |(val,part),i|
    val += part.to_i * MAX_SCORE_PART**(score.length-i-1)
  end
end