What is a hash (HASH) in Ruby?

Hash in Ruby is an ordered collection of key-value pairs, where each unique key is associated with a specific value. Keys are usually symbols (Symbol) or strings (String), but can be any objects that implement the eql? and hash methods.

Features of hash:

• Orderliness: Since Ruby 1.9, hashes maintain the insertion order of elements.
• Access by key: Values are accessed quickly via the corresponding key.
• Dynamic size: Hashes can expand or contract as elements are added or removed.
• No duplicate keys: Each key is unique within a hash.

Examples of creating a hash:

# Using a literal
person = { "name" => "Alice", "age" => 30 }

# Using symbols as keys (more common and efficient way)
settings = { :theme => "dark", :language => "ru" }

# Key-Value Pair Shorthand notation, with Ruby 1.9
config = { host: "localhost", port: 8080 }

# Using Hash.new constructor
empty_hash = Hash.new

Main operations with hashes:

Operation	Example	Description
Access value	person["name"]	Returns the value for the key "name"
Add/modify	person["city"] = "New York"	Adds a pair or modifies the value for the key
Delete	person.delete("age")	Removes the pair with the key "age"
Size	person.size or person.length	Returns the number of pairs in the hash
Check key	person.key?("name")	Checks if the key exists in the hash
Check value	person.value?(30)	Checks if the value exists in the hash
Iteration	`person.each {	key, value
Return keys	person.keys	Returns an array of all keys
Return values	person.values	Returns an array of all values

Usage example:

user_data = {
  name: "Boris",
  age: 45,
  city: "Moscow"
}

# Accessing data
puts user_data[:name]   # => Boris

# Adding a new field
user_data[:job] = "Developer"

# Changing a value
user_data[:age] = 46

# Removing a field
user_data.delete(:city)

# Iterating over the hash
user_data.each do |key, value|
  puts "#{key}: #{value}"
end