Look up values by key

Learn

A Hash associates keys with values. A key identifies an entry without relying on its position. Write a hash between braces; in {name: "Ari"}, name: creates the Symbol key :name.

Use hash[key] to read a value and hash[key] = value to add or update an entry. A Symbol such as :name is a name value, distinct from the String "name". Use the same kind of key for writing and reading.

For the ordinary hashes in this lesson, an absent key returns nil. key?(key) checks whether an entry exists, including an entry whose stored value is nil.

Example

booking = {name: "Ari", seats: 2}
puts booking[:name]
booking[:seats] = 3
puts booking[:seats]

Experiment

• Try Update stock

  Predict the remaining stock. Change the sale from 2 items to 4 while keeping the lookup and update attached to the same key.

• Try Match the key type

  Predict both lookups. Change the second lookup to use a Symbol and compare the results.

• Try Distinguish empty from absent

  Predict the checks. Add profile[:nickname] = "Ace" and see whether the presence check or only the value changes.

Explore the official manual

• Official Ruby 3.2 documentation: Hash
• Official Ruby 3.2 documentation: Literals