Default values for missing keys in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens when you access a missing key in a Ruby hash without a default value?

Ruby returns nil when you access a key that does not exist in the hash and no default value is set.

beginner

How do you set a default value for missing keys in a Ruby hash?

You can set a default value by passing it when creating the hash, like Hash.new(default_value). This value is returned for any missing key.

intermediate

What is the difference between Hash.new(default_value) and hash[key] ||= default_value?

Hash.new(default_value) sets a default for all missing keys automatically. hash[key] ||= default_value sets the value only when you access a missing key and want to assign a default at that moment.

intermediate

How can you use a block to set default values dynamically in a Ruby hash?

You can create a hash with a block like Hash.new { |hash, key| hash[key] = some_value }. This block runs when a missing key is accessed, allowing dynamic default values.

advanced

Why might using a mutable object as a default value cause unexpected behavior?

If you use a mutable object (like an array) as a default value, the same object is shared for all missing keys, so changes affect all keys. Using a block to create a new object each time avoids this.

What does Hash.new(0) do in Ruby?

A Raises an error for missing keys

B Creates a hash with one key 0

C Returns nil for missing keys

D Returns 0 for any missing key

Using Hash.new(0) sets the default value to 0, so any missing key returns 0.

How do you create a hash that assigns a new empty array for each missing key?

A Hash.new([])

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

C Hash.new(nil)

D Hash.new(0)

Using a block with Hash.new { |h, k| h[k] = [] } creates a new array for each missing key, avoiding shared mutable objects.

What will this code output? h = Hash.new('default'); puts h[:missing].object_id == h[:another_missing].object_id

A nil

B false

C true

D Error

The default object is shared, so both missing keys return the same object, making their object_ids equal.

Which method sets a default value only when a missing key is accessed and assigned?

A hash[key] ||= default

B Hash.new(default)

C hash.fetch(key, default)

D hash.default = default

hash[key] ||= default assigns the default value only if the key is missing or falsey.

What is the default return value of a Ruby hash for a missing key if no default is set?

A nil

B empty string

C false

D 0

By default, Ruby hashes return nil for keys that do not exist.

Explain how to set and use default values for missing keys in a Ruby hash.

Think about how Ruby handles missing keys and how you can customize that.

Describe why using a mutable object as a default value can cause problems and how to avoid it.

Consider what happens if you change the default object.