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Ruby Hashes

A hash is a collection that pairs each key with one value. A program reads a value by naming its key, not by counting places.

Pairs with string keys

Each pair joins a name to an age with the => sign. p prints the hash as one value.

ages = { "Ana" => 30, "Bo" => 25 }
p ages

Reading a value by its key

Square brackets hold the key Ana. The printed number is the age stored under that key.

ages = { "Ana" => 30, "Bo" => 25 }
puts ages["Ana"]

A missing key gives nil

The key Cy is not in the hash. p shows nil as the result of that read.

ages = { "Ana" => 30, "Bo" => 25 }
p ages["Cy"]

Adding and changing a pair

A new key adds a pair, and a known key changes its value. p shows the hash after both writes, and the last line counts its pairs.

ages = { "Ana" => 30, "Bo" => 25 }
ages["Cy"] = 40
ages["Bo"] = 26
p ages
puts ages.length

Example

prices = { "pen" => 2, "book" => 8 }
prices["bag"] = 15
prices["pen"] = 3
puts prices["book"]
puts prices.length
p prices

Key points

  • Curly braces hold the pairs of a hash.
  • The => sign joins one key to one value.
  • Each key appears only once in a hash.
  • Square brackets with a key read the value under that key.
  • Reading a key that is not in the hash gives nil.
  • length counts how many pairs a hash holds.

In this example, the prices hash starts with two pairs. A new pair for bag is added, and the price under pen changes to 3. The program then prints the price of book, counts the pairs, and prints the whole hash.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz Ruby Compiler

Exercise

The hash is ages = { "Ana" => 30 }. What does p ages["Bo"] print?