Hashes

A hash stores key-value pairs: you look up a value by its key instead of by its position. Other languages call this a dictionary or map.

Real-world analogy: a hash is like a contacts list. You don't find a phone number by its position in the list — you look it up by the person's name.

Syntax

user = { name: "Ada", age: 36, city: "London" }

name: is shorthand for a symbol key (:name). This is the standard style. Keys can also be strings, written with => ("hash rocket"):

prices = { "apple" => 0.5, "banana" => 0.25 }

Reading and changing values

user = { name: "Ada", age: 36 }

p user[:name]
p user[:email]
p user.fetch(:email, "no email")

user[:email] = "ada@example.com"
user[:age] += 1
user.delete(:name)
p user

p user.key?(:age)
p user.keys
p user.values
p user.size

"Ada"
nil
"no email"
{age: 37, email: "ada@example.com"}
true
[:age, :email]
[37, "ada@example.com"]
2

• A missing key returns nil with [].
• fetch lets you give a default — or, with no default, raises a KeyError so a missing key can't slip through unnoticed.

Iterating

stock = { apples: 10, pears: 0, plums: 4 }

stock.each do |fruit, count|
  puts "#{fruit}: #{count}"
end

apples: 10
pears: 0
plums: 4

Hashes keep the order in which keys were added.

Common mistakes

• Mixing string and symbol keys: a hash built with name: won't find user["name"]. Pick one style per hash.
• Reading a missing key with [] and getting a silent nil. Use fetch when the key must be there.

Practice

Create a hash of three countries and their capitals, add a fourth, then print each pair as France → Paris.

capitals = { "France" => "Paris", "Japan" => "Tokyo", "Peru" => "Lima" }
capitals["Kenya"] = "Nairobi"

capitals.each do |country, capital|
  puts "#{country} → #{capital}"
end