hashes.rb

# http://ruby-doc.org/core/classes/Hash.html

# =====  Initializing  =====
# populated
{ 1 => "one", 2 => "two" } # => {1=>"one", 2=>"two"}
Hash[1, "one", 2, "two"] # => {1=>"one", 2=>"two"}

# empty
{} # => {}
Hash.new # => {}

# =====  Nonexistent Keys  =====
# by default, nonexistent keys return nil
hash = Hash.new # => {}
hash[:josh] # => nil

# to change this behaviour, you can pass new a block that receives the hash and the nonexistent key
# and then adds it to the hash in the way that you would like
aliases = Hash.new { |this_hash, key| this_hash[key] = Array.new }
aliases # => {}
aliases[:josh] # => []
aliases # => {:josh=>[]}
aliases[:jeff] << "the dude" << "his dudeness" << "duder" << "el duderino"
aliases # => {:josh=>[], :jeff=>["the dude", "his dudeness", "duder", "el duderino"]}

# =====  Access and Assign  =====
# generally access and assign like ths
hash = Hash.new
hash # => {}
hash["my key"] = "my value"
hash # => {"my key"=>"my value"}
hash["my key"] # => "my value"

# access multiple values at once
hash = Hash[*[Array("a".."j"), Array(1..10)].transpose.flatten] # just populating
hash # => {"a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5, "f"=>6, "g"=>7, "h"=>8, "i"=>9, "j"=>10}
hash.values_at "j", "e", "i", :x # => [10, 5, 9, nil]

# more interesting options with fetch (think how this might be helpful in param default values)
hash = { "a" => 100, "b" => 200 }
hash.fetch "a" # => 100
hash.fetch "z", "go fish" # => "go fish"
hash.fetch("z") { |el| "go fish, #{el}" } # => "go fish, z"

# sometimes you might use has_key to check for a key, for example, if you're using an init block
hash = Hash.new { true }
hash[:a] = :b
hash[1000] # => true
hash.has_key? 1000 # => false

hash = { 12 => nil }
hash[12] # => nil
hash[13] # => nil
hash.has_key? 12 # => true
hash.has_key? 13 # => false

# and the converse
hash = { "a" => 100, "b" => 200 }
hash.has_value?(100) # => true
hash.has_value?(999) # => false

# =====  Removal  =====
# delete accepts a key and returns its value. You can give a block that will be invoked and returned
# in the event that the keyt o delete was not seen
hash = Hash[*Array(1..10)] # => {1=>2, 3=>4, 5=>6, 7=>8, 9=>10}
hash.delete 1 # => 2
hash.delete 100 # => nil
hash # => {3=>4, 5=>6, 7=>8, 9=>10}
hash.delete(3) { |n| "#{n} not found" } # => 4
hash.delete(100) { |n| "#{n} not found" } # => "100 not found"

# delete_if to remove all pairs from a hash
# that cause the block to evaluate to true
hash = Hash[*Array(1..10)] # => {1=>2, 3=>4, 5=>6, 7=>8, 9=>10}
hash.delete_if { |key, value| key % 5 == 0 || value % 5 == 0 }
hash # => {1=>2, 3=>4, 7=>8}

# =====  Iterating  =====
# pass a block that receives the key and the value
cities = { Chicago: :USA, Paris: :France, Ramstein: :Germany }
results = Array.new
cities.each { |city, country| results << "#{city} is in #{country}" }
results # => ["Chicago is in USA", "Paris is in France", "Ramstein is in Germany"]

# =====  Other Useful Methods  =====

# there is also a self mutator version: merge!
defaults = { color: :red, city: "Wichita" }
custom = { city: "Boston" }
merged = defaults.merge custom
defaults # => {:color=>:red, :city=>"Wichita"}
custom # => {:city=>"Boston"}
merged # => {:color=>:red, :city=>"Boston"}

cities = { Chicago: :USA, Paris: :France, Ramstein: :Germany }
cities.keys # => [:Chicago, :Paris, :Ramstein]
cities.values # => [:USA, :France, :Germany]
cities.length # => 3
cities.size # => 3
cities.empty? # => false
Hash.new.empty? # => true

hash = Hash[*Array(1..10)]
hash # => {1=>2, 3=>4, 5=>6, 7=>8, 9=>10}
hash.clear
hash # => {}

# get a key for a given value (remember, there could be more than one key with the same value)
hash = { "a" => 100, "b" => 200 }
hash.index(200) # => "b" # !> Hash#index is deprecated; use Hash#key
hash.index(999) # => nil # !> Hash#index is deprecated; use Hash#key

# swap keys with values
hash = { n: 1, m: 1, o: 2 } # n and m have the same value, n ends up getting squashed
hash.invert # => {1=>:m, 2=>:o}

# convert to an array
{ Chicago: :USA, Paris: :France, Ramstein: :Germany }.to_a # => [[:Chicago, :USA], [:Paris, :France], [:Ramstein, :Germany]]