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!

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– Andrey Deineko, Commented Jan 10, 2017 at 5:16
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– Rob Matthews, Commented Mar 21, 2017 at 16:49

Andrey Deineko · Accepted Answer · 2017-01-10 07:48:06Z

4

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.

edited Jan 10, 2017 at 7:48

answered Jan 10, 2017 at 4:35

Andrey Deineko

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7 Comments

tadman Over a year ago

Using instance_variable_get is a very rude thing to do from an object-oriented design perspective, you're messing around with another object's private data. The attr_reader approach here is the best, and really only way to solve this properly. The latter's a last-ditch emergency option.

Andrey Deineko Over a year ago

@tadman posted it almost exclusively for showing alternative option (for example, if user does not have a chance to add attr_reader to Orange), but I agree that instance_variable_get is definitely not a way to go here..

tadman Over a year ago

Yeah, I agree, just flagging that in case people get ideas.

Ronats Over a year ago

How can we check "oranges_array" carries array of objects? Like oranges_array.is_a? like anything is there

Andrey Deineko Over a year ago

@Ronats I don't understand what exactly you are trying to ask/do. Could you precise so I can try helping you out?

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