In Ruby, how can I get instance variables in a hash instead of an array?

Question

I have a Ruby class. I want to get an instance variable from an argument to a method in that class. I can do get all of the instance variables as an array:

self.instance_variables

However, I want to get the instance variable named arg, specifically:

class MyClass
  def get_instance_variable(arg)
    hash_of_instance_variables[arg]
  end
end

object.get_instance_variable('my_instance_var')

How do I compute hash_of_instance_variables?

It looks like you're hoping to build a hash that maps instance variables to their values so that you can retrieve the value of the instance variable. If so, why? Use `instance_variable_get`. — cam, Feb 02 '11 at 03:17

score 24 · Accepted Answer · edited Dec 07 '11 at 15:39

24

To create a hash of all instance variables you can use the following code:

class Object
  def instance_variables_hash
    Hash[instance_variables.map { |name| [name, instance_variable_get(name)] } ]
  end
end

But as cam mentioned in his comment, you should use instance_variable_get method instead:

object.instance_variable_get :@my_instance_var

edited Dec 07 '11 at 15:39

Michael Kohl

66,324
14
138
158

answered Dec 07 '11 at 14:55

Yossi

11,778
2
53
66

score 13 · Answer 2 · answered Oct 20 '14 at 13:14

13

Question is quite old but found rails solution for this: instance_values

This is first answer in google so maybe it will help someone.

answered Oct 20 '14 at 13:14

sufleR

2,865
17
31

Thanks for helping the community. I no longer use ruby/rails, moved on to node.js. Bests – Totty.js Oct 20 '14 at 13:35
The code used by Rails is almost identical to the accepted answer with the exception that Rails strips the leading `@`. – Michael Mior May 07 '15 at 17:54
`instance_values` worked for me and was simpler. Thanks! (now tested with Rails 6) – Jesper Rønn-Jensen Aug 31 '20 at 07:29

score 1 · Answer 3 · answered Dec 07 '11 at 14:05

1

class MyClass    
def variables_to_hash
      h = {}
      instance_variables.each{|a|
        s = a.to_s
        n = s[1..s.size]
        v = instance_variable_get a
        h[n] = v
      }
      h
    end
end

answered Dec 07 '11 at 14:05

agluszak

90
1
6

Note that `s[1..s.size]` can be replaced with `s[1..-1]`. – Michael Mior May 07 '15 at 17:55

baka-san · Answer 4 · 2022-12-20T06:41:10.843

For Ruby 2.6+, you can pass a block to the to_h method, leading to very DRY syntax:

# Some instance variables
instance_variable_set(:@a, 'dog')
#=> "dog"
instance_variable_set(:@b, 'cat')
#=> "cat"

# Array of instance variable names
instance_variables
#=> [:@a, :@b]

# Hash of instance variable names and values, including leading @
instance_variables.to_h { |k| [k, instance_variable_get(k)] }
#=> { :@a => "dog", :@b => "cat" }

# Hash of instance variable names and values, excluding leading @
instance_variables.to_h { |k| [k[1..-1].to_sym, instance_variable_get(k)] }
#=> { :a => "dog", :b => "cat" }

score -1 · Answer 5 · edited Jun 20 '20 at 09:12

-1

Ruby on Rails has a couple of built-in ways to do this that you might find meet your needs.

user = User.new(first: 'brian', last: 'case')

attributes (Rails API: ActiveModel::AttributeMethods)

user.attributes

{"first"=>"brian", "last"=>"case"}

serializable_hash (Rails API: Active Model Serialization)

user.serializeable_hash

{"first"=>"brian", "last"=>"case"}

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 25 '16 at 22:59

briancase

11
3

This only applies to Rails models (instances of `ActiveRecord::Base`). For an arbitrary Ruby object, @sufleR's answer works. – Simon Lepkin Aug 17 '16 at 16:18

In Ruby, how can I get instance variables in a hash instead of an array?

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