Access instance variable values in array of objects: Ruby

Question

I have an array of objects with one instance variable:

oranges_array = 
[#<Orange:0x007faade859ea0 @diameter=2.7>,
 #<Orange:0x007faade859ef0 @diameter=2.8>,
 #<Orange:0x007faade859f40 @diameter=2.6>]

For example, how would I access the diameter instance variables? I want to eventually get their average. Do I need to or am I on the right track in thinking I maybe use an each loop to shovel each diameter into its own array then use inject? Or is there a simpler way to access the diameter variables? Thanks!

Answer 1

One option is to add attr_reader to Orange class:

class Orange
  attr_reader :diameter
end

Now you can get the avarage:

avg = oranges_array.map(&:diameter).inject(:+) / oranges_array.size.to_f
# with Ruby 2.4+ you can do the following:
# avg = oranges_array.sum(&:diameter) / oranges_array.size.to_f

Another option is to use Object#instance_variable_get (without having to add the attr_reader to the class:

avg = oranges_array.map { |orange| orange.instance_variable_get(:@diameter) }.inject(:+) / oranges_array.size.to_f

NOTE:

Using instance_variable_get for this task is the last resort option and I added it almost exclusively to show there's a way to get instance variables of objects in case there is, for example, no way to define attribute readers in the class.