Hashes of Hashes Idiom in Ruby?

Question

Creating hashes of hashes in Ruby allows for convenient two (or more) dimensional lookups. However, when inserting one must always check if the first index already exists i

Answer 1

Autovivification, as it's called, is both a blessing and a curse. The trouble can be that if you "look" at a value before it's defined, you're stuck with this empty hash in the slot and you would need to prune it off later.

If you don't mind a bit of anarchy, you can always just jam in or-equals style declarations which will allow you to construct the expected structure as you query it:

((h ||= { })['w'] ||= { })['z']

Answer 2

You can pass the Hash.new function a block that is executed to yield a default value in case the queried value doesn't exist yet:

h = Hash.new { |h, k| h[k] = Hash.new }

Of course, this can be done recursively. There's an article explaining the details.

For the sake of completeness, here's the solution from the article for arbitrary depth hashes:

hash = Hash.new(&(p = lambda{|h, k| h[k] = Hash.new(&p)}))

The person to originally come up with this solution is Kent Sibilev.

Answer 3

require 'xkeys'
h = {}.extend XKeys::Hash

h['x', 'y'] = value_to_insert
looked_up_value = h['w', 'z'] # nil

h[:foo, :bar, :baz, :else => 0] += 1