Hash#invert

319 views

What's the return value of this Ruby 3 code?

h = { a: 1, b: 2 }
h.replace(h.invert)

h[:a] # => ???

The correct answer is

2

:b

It raises NoMethodError

nil

Explanation

Initial Hash:

h = { a: 1, b: 2 }

Here, h is a hash with two key-value pairs:

Key :a maps to value 1
Key :b maps to value 2

The `invert` Method:

The invert method swaps the keys and values in the hash. For h = { a: 1, b: 2 }, calling invert results in:

h.invert # => { 1 => :a, 2 => :b }

After inversion:

The original key :a (which mapped to 1) is now the value, and 1 becomes the key.
Similarly, :b (which mapped to 2) is now the value, and 2 becomes the key.

The `replace` Method:

h.replace(h.invert) # => { 1 => :a, 2 => :b }

The replace method completely replaces the contents of the original hash h with the contents of another hash.

In this case, the original h (which was { a: 1, b: 2 }) is replaced by the result of h.invert (which is { 1 => :a, 2 => :b }).

Now, the original key :a no longer exists in the hash because it was replaced by 1 in the inversion process.

So, when you try to access h[:a], it returns nil because there is no key :a in the new hash.

RubyCademy ©