Suppose you have a class written Ruby and you want to sort an array of instances of that class:

class Player
  attr_accessor :name, :avg, :hr, :rbi

  def initialize(name, avg, hr, rbi)
    @name = name
    @avg = avg
    @hr = hr
    @rbi = rbi
  end
end

players = [
  Player.new('Dominic Smith', 0.253, 11, 43),
  Player.new('Brandon Nimmo', 0.299, 2, 14),
  Player.new('Michael Conforto', 0.203, 6, 24)
]

According to this blog post by Brandon Dimcheff, there are two methods built into Ruby arrays for sorting: sort and sort_by.

The sort method calls a comparator block that you define. The block compares two objects at a time and it must return -1 the first item is less than the second, 0 if the two objects are equal, or 1 if the first item is greater than the second one.

players.sort { |first_player, second_player| first_player.avg <=> second_player.avg }

The sort_by methods also calls a block, but in this case the block only takes in one argument. The array is then sorted by value returned by that block. sort_by uses a Schwartzian Transform which turns each element into a tuple with the value you want to sort by as the first element in each tuple, then the tuples are sorted by that first value. sort_by may result in faster performance, especially when the comparison between two objects is expensive, since the operator only gets called once for each element.

players.sort_by { |player| player.avg }

In both cases, errors arise when the method tries to compare values that cannot be compared – namely nil. When this happens, execution stops and an exception is thrown. I had to debug such an issue at work, where an object had a nil value in one of its fields. I can’t share the data I used at work, but I can replace it with this example:

players = [
  Player.new('Dominic Smith', 0.253, 11, 43),
  Player.new('Brandon Nimmo', 0.299, 2, 14),
  Player.new('Michael Conforto', nil, 6, 24)
]

When calling sort again, this happens:

irb(main):016:0> players.sort { |first_player, second_player| first_player.avg <=> second_player.avg }
Traceback (most recent call last):
        5: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `<main>'
        4: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `load'
        3: from /Users/creativedan/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        2: from (irb):16
        1: from (irb):16:in `sort'
ArgumentError (comparison of Player with Player failed)

The error, while incidating that there is a problem with the comparison, doesn’t explain why the comparison failed. Let’s see what happens when we call sort_by:

irb(main):017:0> players.sort_by { |player| player.avg }
Traceback (most recent call last):
        6: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `<main>'
        5: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `load'
        4: from /Users/creativedan/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        3: from (irb):17
        2: from (irb):17:in `rescue in irb_binding'
        1: from (irb):17:in `sort_by'
ArgumentError (comparison of Float with nil failed)

That’s slightly better, since it at least shows that the nil value is the one at fault.

The error becomes more obscure when sorting objects by multiple fields. This can be done by calling sort_by and having the block return an array of values. However, the sort errors out when any of the values in the array is nil:

irb(main):018:0> players.sort_by { |player| [player.avg, player.hr] }
Traceback (most recent call last):
        6: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `<main>'
        5: from /Users/creativedan/.rbenv/versions/2.6.3/bin/irb:23:in `load'
        4: from /Users/creativedan/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        3: from (irb):18
        2: from (irb):18:in `rescue in irb_binding'
        1: from (irb):18:in `sort_by'
ArgumentError (comparison of Array with Array failed)

It’s that specific error (comparison of Array with Array failed) that I encountered in a teammate’s code. Fortunately, Tomer Brisker on Medium encountered a similar issue in his codebase and provided a key clue on getting the error resolved.

If you want to sort arrays without a hitch, you’ll need to do either of the following:

  • Ensure there are no nils in your input
  • Handle nil values in your comparator block

Most importantly, you need to be aware that nils will cause your sorting method to crash.

I wrote a Gist if you’re interested in running the code examples above.