Ruby Arrays of Objects and Unions

less than 1 minute read

I’m working through the Advent of Code and needed to union two Ruby arrays of objects together based on some properties on said objects. I wasn’t having much luck getting it to work and my Google fu was failing but I finally figured out the issue and want to post it here in case someone else ever manages to search using the right terms.

The key here is that you can’t just override eql?. You have to also override the hash method. So for a Position class, it might look like this:

<pre class="lang:ruby decode:true " title="Overriding eql? and hash">class Position
  attr_reader :x, :y

  def hash
    [@x, @y].hash

  def initialize(coord=[])
    if coord==[]
      @x = 0
      @y = 0
      @x = coord[0]
      @y = coord[1]

  def eql?(other_object)
    @x == other_object.x && @y == other_object.y

This would allow the union of two arrays of Position objects to only include Positions that are unique by X and Y coordinates.