Creating a deeply nested Hash in Ruby

Ruby is a very dynamic programming language. Here is a quick demonstration of this fact.

It’s not something I need on a daily basis but it happens that I want to be able to create a hash in which I could set or read the value for an arbitrary deep key without even knowing if its ancestor keys exists.

Common hash usage

Problem

Let’s say I have a variable named hash that contains a hash:

irb> hash = {}

and I want to know what is the value for hash[:foo][:bar][:baz]:

irb> hash[:foo][:bar][:baz]
NoMethodError: undefined method `[]' for nil:NilClass

That was pretty predictable. But how do I read these nested values?

To go further, how do I assign a value to some deep key?

irb> hash[:foo][:bar][:baz] = "hello"
NoMethodError: undefined method `[]=' for nil:NilClass

The same kind of exception is raised.

Solution

It’s time to create this deep hash by hand and it’s gonna be painful:

irb> hash = {}
=> {}
irb> hash[:foo] ||= {}
=> {}
irb> hash[:foo][:bar] ||= {}
=> {}
irb> hash[:foo][:bar][:baz] = "hello"
=> "hello"
irb> hash
=> {:foo=>{:bar=>{:baz=>"hello"}}}

I’m pretty sure you don’t like to write this kind of thing in your code, do you?

When I’m facing something like that I can’t stop thinking that there’s a better way to do it since Ruby was designed with programmers happiness in mind.

Next level hash usage

Obviously there’s a better way!

Most of the time in Ruby we create our hashes by using the syntax we saw above. That’s fine for common needs but you must know that this syntax is only a syntactic sugar to quickly create a hash but under the hood it’s a call to the Hash.new method.

If you read Hash.new documentation carefully you’ll notice that there is a signature that allows to pass a block to the Hash.new call.

This block let you choose how the hash should act when you try to access a key that doesn’t exist. This is exactly what we need to implement our infinitely deep nested hash!

Making a hash dynamic

Let’s do it:

irb> hash = Hash.new { |h, k| h[k] = h.dup.clear }
=> {}
irb> hash[:foo][:bar][:baz] = "test"
=> "test"
irb> hash["access"]["only"]
=> {}
irb> hash
=> {:foo=>{:bar=>{:baz=>"test"}}, "access"=>{"only"=>{}}}

Much better!

As you can see we can now assign a value to an arbitrarily deeply nested key in the hash without having to take care of creating ancestors. That’s pretty awesome, isn’t it?

You may ask what’s the sorcery done in our example new block?

This is kind of recursion in a way. By using h[k] = h.dup.clear we are telling that if the key k is missing we want to duplicate h, which is our main hash, then clear its content and store it as the value of the k key.

It can seem a bit weird but actually it’s not. This cloning / clearing black magic creates a new dynamic hash as the value for this key by reusing our main hash behavior.

Gotcha

As you have may notice there’s a gotcha you have to be aware of when using this trick.

As soon as you’ll try to access a given key in the hash, it will create the key with content being what you’ve defined in the new block.

That’s what happened with the hash["access"]["only"] call in our example.

So if you assign a value to a non-existing key, this key and all its missing ancestors will be created and the value will be associated to the key.

But if you try to read the value of a non-existing key, without setting its content, then this key will be created with a value defaulting to what was defined in the block. In our example it’s going to be an empty hash.

Hope that this little tip will help you someday. Have fun and stay safe.