How to access a symbol hash key using a variable in Ruby

Question

I have an array of hashes to write a generic checker for, so I want to pass in the name of a key to be checked. The hash was defined with keys with symbols (colon prefixes). I can't figure out how to use the variable as a key properly. Even though the key exists in the hash, using the variable to access it results in nil.

In IRB I do this:

>> family = { 'husband' => "Homer", 'wife' => "Marge" }
=> {"husband"=>"Homer", "wife"=>"Marge"}
>> somevar = "husband"
=> "husband"
>> family[somevar]
=> "Homer"
>> another_family  = { :husband => "Fred", :wife => "Wilma" }
=> {:husband=>"Fred", :wife=>"Wilma"}
>> another_family[somevar]
=> nil
>>

How do I access the hash key through a variable? Perhaps another way to ask is, how do I coerce the variable to a symbol?

Solution 1

You want to convert your string to a symbol first:

another_family[somevar.to_sym]

If you want to not have to worry about if your hash is symbol or string, simply convert it to symbolized keys

see: How do I convert a Ruby hash so that all of its keys are symbols?

Solution 2

You can use the Active Support gem to get access to the with_indifferent_access method:

require 'active_support/core_ext/hash/indifferent_access'
> hash = { somekey: 'somevalue' }.with_indifferent_access
 => {"somekey"=>"somevalue"}
> hash[:somekey]
 => "somevalue"
> hash['somekey']
=> "somevalue"

Solution 3

Since your keys are symbols, use symbols as keys.

> hash = { :husband => 'Homer', :wife => 'Marge' }
 => {:husband=>"Homer", :wife=>"Marge"}
> key_variable = :husband
 => :husband
> hash[key_variable]
 => "Homer"

Comments

[Cary Swoveland]

"husband".to_sym => :husband.

[Ken Ratanachai S.]

@CarySwoveland Watch out for NoMethodError: undefined method `to_sym' for nil:NilClass when var is nil

[PJP]

This uses ActiveSupport Core Extensions, which is the proper way to cherry-pick specific extensions.

[Leonard]

This might work in some circumstances, but in my case I'm passing them in as parameters, and Trollop (argument parsing gem) won't accept a symbol as a default value.

[Ken Ratanachai S.]

Do watch out for nil somevar, though.