Iterators and Enumerables

Hashes have a different set of iterators than arrays. They share each, but hashes also have the methods each_key and each_value. Hashes don't have an each_index method because they don't have indices! Because hashes store key-value pairs rather than individual elements, the syntax for iterating through them using each differs: the given block takes two arguments, a key and a value.

translations = {
  Latin: "suum cuique",
  English: "to each his own",
  Russian: "каждому свое",
  Spanish: "A cada cual lo suyo"
}

# each takes a block that takes two arguments: a key and value
translations.each {|k, v| puts "#{k}: #{v}"}  

# each_key and each_value take a block that takes one argument: a key or a value
translations.each_key {|k| puts k}
translations.each_value {|v| puts v}

Hashes have access to all the enumerables you've hopefully grown to love except for each_with_index and with_index. Like each, the given block takes two arguments--a key and a value--when the receiver is a hash.

silly_yllis = {
  "stressed"=>"desserts",
  "redrum"=>"murder",
  "god"=>"dog",
  "erehwon"=>"nowhere"
}

silly_yllis.all? {|k, v| k == v.reverse} #=> true
silly_yllis.count {|k, v| k != v.reverse} #=> 0

# map still returns an array
silly_yllis.map {|k, v| k + v} #=> ["stresseddesserts", "redrummurder", "goddog", "erehwonnowhere"]

# select returns a hash
silly_yllis.select {|k, v| k.length < 4} #=> {"god" => "dog"}

With a hash as the receiver, sort_by returns a nested array of [key, value] arrays in the order specified by its block:

  to_be_sorted_by_key = {b: 5, a: 10, c: 20}
  to_be_sorted_by_key.sort_by {|k, v| k} #=> [[:a, 10], [:b, 5], [:c, 20]]