Best way to convert strings to symbols in hash

Question

What\'s the (fastest/cleanest/straightforward) way to convert all keys in a hash from strings to symbols in Ruby?

This would be handy when parsing YAML.

Answer 1

You could be lazy, and wrap it in a lambda:

my_hash = YAML.load_file('yml')
my_lamb = lambda { |key| my_hash[key.to_s] }

my_lamb[:a] == my_hash['a'] #=> true

But this would only work for reading from the hash - not writing.

To do that, you could use Hash#merge

my_hash = Hash.new { |h,k| h[k] = h[k.to_s] }.merge(YAML.load_file('yml'))

The init block will convert the keys one time on demand, though if you update the value for the string version of the key after accessing the symbol version, the symbol version won't be updated.

irb> x = { 'a' => 1, 'b' => 2 }
#=> {"a"=>1, "b"=>2}
irb> y = Hash.new { |h,k| h[k] = h[k.to_s] }.merge(x)
#=> {"a"=>1, "b"=>2}
irb> y[:a]  # the key :a doesn't exist for y, so the init block is called
#=> 1
irb> y
#=> {"a"=>1, :a=>1, "b"=>2}
irb> y[:a]  # the key :a now exists for y, so the init block is isn't called
#=> 1
irb> y['a'] = 3
#=> 3
irb> y
#=> {"a"=>3, :a=>1, "b"=>2}

You could also have the init block not update the hash, which would protect you from that kind of error, but you'd still be vulnerable to the opposite - updating the symbol version wouldn't update the string version:

irb> q = { 'c' => 4, 'd' => 5 }
#=> {"c"=>4, "d"=>5}
irb> r = Hash.new { |h,k| h[k.to_s] }.merge(q)
#=> {"c"=>4, "d"=>5}
irb> r[:c] # init block is called
#=> 4
irb> r
#=> {"c"=>4, "d"=>5}
irb> r[:c] # init block is called again, since this key still isn't in r
#=> 4
irb> r[:c] = 7
#=> 7
irb> r
#=> {:c=>7, "c"=>4, "d"=>5}

So the thing to be careful of with these is switching between the two key forms. Stick with one.

Answer 2

In ruby I find this to be the most simple and easy to understand way to turn string keys in hashes to symbols :

my_hash.keys.each { |key| my_hash[key.to_sym] = my_hash.delete(key)}

For each key in the hash we call delete on it which removes it from the hash (also delete returns the value associated with the key that was deleted) and we immediately set this equal to the symbolized key.

Answer 3

Even more terse:

Hash[my_hash.map{|(k,v)| [k.to_sym,v]}]

Answer 4

a shorter one-liner fwiw:

my_hash.inject({}){|h,(k,v)| h.merge({ k.to_sym => v}) }

Answer 5

I like this one-liner, when I'm not using Rails, because then I don't have to make a second hash and hold two sets of data while I'm processing it:

my_hash = { "a" => 1, "b" => "string", "c" => true }

my_hash.keys.each { |key| my_hash[key.to_sym] = my_hash.delete(key) }

my_hash
=> {:a=>1, :b=>"string", :c=>true}

Hash#delete returns the value of the deleted key

Answer 6

Similar to previous solutions but written a bit differently.

• This allows for a hash that is nested and/or has arrays.
• Get conversion of keys to a string as a bonus.

• Code does not mutate the hash been passed in.

  module HashUtils
    def symbolize_keys(hash)
      transformer_function = ->(key) { key.to_sym }
      transform_keys(hash, transformer_function)
    end
  
    def stringify_keys(hash)
      transformer_function = ->(key) { key.to_s }
      transform_keys(hash, transformer_function)
    end
  
    def transform_keys(obj, transformer_function)
      case obj
      when Array
        obj.map{|value| transform_keys(value, transformer_function)}
      when Hash
        obj.each_with_object({}) do |(key, value), hash|
          hash[transformer_function.call(key)] = transform_keys(value, transformer_function)
        end
      else
        obj
      end
    end
  end