Ruby - defining class level variable dynamically

Question

I am new to ruby and wanted to know if this is possible. Suppose I have a file with different blocks like this

fruits[tomato=1,orange=2]
greens[peas=2,potato=3]

I have parsed this file and stored it into a hash like this

{"fruits"=>{"tomato"=>"1", "orange"=>"2"}, "greens"=>{"potato"=>"3", "peas"=>"2"}}

And I also know how to access the different parts of the hash. But suppose if want to make it something like this

fruits.tomato  # 1
fruits.orange  # 2
(Like an object with tomato and orange being its variables)

The catch here is suppose I don't know if the file is going to contain fruits and greens, it could contain a different group called meat. I know this dynamic problem can be solved if I insert everything into a hash with the key as group name and value will be another hash. But can this be done with the example of fruit.tomato or fruits.orange I provided above(Probably by declaring it in a class but I am not sure how to dynamically add class vars in ruby or if that is even possible as I am new to the language).

Anthony · Accepted Answer · 2015-02-20 01:10:22Z

1

I spent quite a bit of time making a program just like this in order to help speed up development with API's. I ended up writing a gem to objectify raw JSON (shameless plug: ClassyJSON).

That said, I think your use case is a good one for OpenStruct. I limited my code to just your example and your desired result but here's what it might look like:

require 'ostruct'

hash = {"fruits"=>{"tomato"=>"1", "orange"=>"2"}, "greens"=>{"potato"=>"3", "peas"=>"2"}}
structs = []

hash.each do |k, v|
  if v.is_a? Hash
    obj = OpenStruct.new({k => OpenStruct.new(v)})
  end
  structs << obj
end

Here we built up a number of OpenStruct objects and can access their values as you outlined:

[1] pry(main)> structs
=> [#<OpenStruct fruits=#<OpenStruct tomato="1", orange="2">>, #<OpenStruct greens=#<OpenStruct potato="3", peas="2">>]
[2] pry(main)> structs.first
=> #<OpenStruct fruits=#<OpenStruct tomato="1", orange="2">>
[3] pry(main)> structs.first.fruits
=> #<OpenStruct tomato="1", orange="2">
[4] pry(main)> structs.first.fruits.tomato
=> "1"

edited Feb 20, 2015 at 1:10

answered Feb 20, 2015 at 1:04

Anthony

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4 Comments

Anthony Over a year ago

cool. You'll want to add some logic to that loop if you have arrays or simple key values but that should get you going.

anonymous123 Over a year ago

Sorry, about that I may not posted my whole question. So do you think OpenStruct is a favorable way to come to this solution as I mean this would be similar to a Struct in c/c++ which I have been told to stay away from.

Anthony Over a year ago

It's been a really long time since I've done any C so I can't really remember the pros and cons. OpenStruct uses ruby's metaprogramming under the hood so it is unique in that sense.

anonymous123 Over a year ago

Anthony I found this solution which I am going to use and can be used directly when I am creating the hashmap. I have added it as an answer.

anonymous123 · Accepted Answer · 2015-02-20 13:52:16Z

0

  def add_accessor_method(name, ref)
    define_singleton_method name do
     return ref
    end
  end

I found this solution which will make an accessor method for me during parsing the file itself. So I will not have to use OpenStruct later on to convert my hash to an object with different accessor methods. ( I am sure OpenStruct under the hood is doing that)

answered Feb 20, 2015 at 13:52

anonymous123

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