Hashes

Ruby Hashes

Hashes are like dictionaries in other languages. They store key-value pairs. Ruby gives you two main ways to create them.

The old syntax uses Hash.new and the "rocket" => operator:

person = Hash.new
person = {"name" => "Alice", "age" => 30}

The newer syntax uses symbols with a colon — cleaner and more idiomatic:

person = {name: "Alice", age: 30}

Access values with the key. Use fetch when you want a safe default if the key doesn't exist:

person[:name]                # => "Alice"
person.fetch(:name)          # => "Alice"
person.fetch(:email, "N/A")  # => "N/A" (default since :email is missing)

You can grab all the keys or values separately, or iterate over them:

person.keys        # => [:name, :age]
person.values      # => ["Alice", 30]
person.each_key    { |k| puts k }
person.each_value  { |v| puts v }

Combine two hashes with merge. If both have the same key, the second hash wins:

defaults = {color: "red", size: "medium"}
user_pref = {color: "blue"}

settings = defaults.merge(user_pref)
# => {:color=>"blue", :size=>"medium"}

Hashes also work beautifully as keyword arguments in methods, which we'll see later.