Tutorial: Hashes in Ruby

Hashes in Ruby are collections of key-value pairs, similar to dictionaries in other programming languages.

They allow you to store and retrieve data efficiently using keys.

What You’ll Learn

1. What Are Hashes?

A hash is a collection of unique keys and their associated values. Keys can be any object (most commonly symbols or strings), and values can be any object.

Example

hash = { name: "Alice", age: 25, city: "New York" }

2. Creating Hashes

2.1 Using Curly Braces

hash = { name: "Bob", age: 30 }
puts hash  # Output: {:name=>"Bob", :age=>30}

2.2 Using the Hash Class

hash = Hash.new
hash[:name] = "Charlie"
puts hash  # Output: {:name=>"Charlie"}

2.3 Default Value

You can specify a default value for missing keys.

hash = Hash.new("Not Found")
puts hash[:missing_key]  # Output: Not Found

3. Accessing Hash Elements

Access Using Keys

person = { name: "Alice", age: 25 }
puts person[:name]  # Output: Alice
puts person[:age]   # Output: 25

Using fetch for Default Values

person = { name: "Alice" }
puts person.fetch(:age, "Unknown")  # Output: Unknown

4. Adding and Updating Elements

Adding New Key-Value Pairs

person = { name: "Alice" }
person[:age] = 25
puts person  # Output: {:name=>"Alice", :age=>25}

Updating Existing Keys

person[:name] = "Bob"
puts person  # Output: {:name=>"Bob", :age=>25}

5. Deleting Elements

Using delete

person = { name: "Alice", age: 25 }
person.delete(:age)
puts person  # Output: {:name=>"Alice"}

6. Iterating Over Hashes

Iterate Through Keys and Values

person = { name: "Alice", age: 25 }
person.each do |key, value|
  puts "#{key}: #{value}"
end
# Output:
# name: Alice
# age: 25

Iterate Through Keys Only

person.each_key { |key| puts key }
# Output:
# name
# age

Iterate Through Values Only

person.each_value { |value| puts value }
# Output:
# Alice
# 25

7. Common Hash Methods

7.1 Check for a Key or Value

person = { name: "Alice", age: 25 }
puts person.key?(:name)     # Output: true
puts person.value?(25)      # Output: true

7.2 Merge Two Hashes

hash1 = { name: "Alice", age: 25 }
hash2 = { city: "New York", age: 30 }
merged = hash1.merge(hash2)
puts merged  # Output: {:name=>"Alice", :age=>30, :city=>"New York"}

7.3 Select Specific Elements

person = { name: "Alice", age: 25, city: "New York" }
adults = person.select { |key, value| key == :age && value >= 18 }
puts adults  # Output: {:age=>25}

7.4 Reject Specific Elements

person = { name: "Alice", age: 25, city: "New York" }
filtered = person.reject { |key, value| key == :city }
puts filtered  # Output: {:name=>"Alice", :age=>25}

7.5 Convert to Array

person = { name: "Alice", age: 25 }
puts person.to_a  # Output: [[:name, "Alice"], [:age, 25]]

7.6 Get Keys or Values

person = { name: "Alice", age: 25 }
puts person.keys   # Output: [:name, :age]
puts person.values # Output: ["Alice", 25]

8. Practical Examples

8.1 Counting Word Frequencies

text = "hello world hello Ruby"
words = text.split
frequencies = Hash.new(0)

words.each { |word| frequencies[word] += 1 }
puts frequencies
# Output: {"hello"=>2, "world"=>1, "Ruby"=>1}

8.2 Grouping by Criteria

people = [
  { name: "Alice", age: 25 },
  { name: "Bob", age: 17 },
  { name: "Carol", age: 30 }
]

grouped = people.group_by { |person| person[:age] >= 18 ? :adult : :minor }
puts grouped
# Output: {:adult=>[{:name=>"Alice", :age=>25}, {:name=>"Carol", :age=>30}], :minor=>[{:name=>"Bob", :age=>17}]}

8.3 Using Hash for Default Configurations

def configure(settings = {})
  defaults = { theme: "light", language: "en", notifications: true }
  config = defaults.merge(settings)
  puts config
end

configure(language: "fr", notifications: false)
# Output: {:theme=>"light", :language=>"fr", :notifications=>false}

8.4 Sorting a Hash by Key or Value

person = { name: "Alice", age: 25, city: "New York" }

# Sort by keys
sorted_by_key = person.sort.to_h
puts sorted_by_key  # Output: {:age=>25, :city=>"New York", :name=>"Alice"}

# Sort by values
sorted_by_value = person.sort_by { |key, value| value }.to_h
puts sorted_by_value  # Output: {:name=>"Alice", :city=>"New York", :age=>25}

8.5 Hash with Default Values for Caching

cache = Hash.new { |hash, key| hash[key] = key ** 2 }
puts cache[2]  # Output: 4
puts cache[3]  # Output: 9
puts cache     # Output: {2=>4, 3=>9}

9. Best Practices for Using Hashes

1. Use Symbols for Keys: Symbols are more memory-efficient and faster than strings as keys.
2. Use Default Values: Set default values to handle missing keys gracefully.
3. Merge Hashes for Configurations: Use merge to combine default and custom configurations.
4. Leverage Iterators: Use each, select, and reject for concise and readable hash operations.

10. Summary

Key Concepts

• Hashes store key-value pairs.
• Access values using keys or methods like fetch.
• Use select, reject, and merge to manipulate hash data.
• Iterating over hashes is straightforward with each, each_key, and each_value.

Examples in Context

Hashes are versatile and powerful for organizing data in Ruby.