Ruby hash recap

hash_recap.rb

h1 = Hash.new          # => {}
h2 = Hash.new("a")     # => {}
h3 = {}                # => {}
h4 = {abc: "def"}      # => {:abc=>"def"}
h5 = {:abc => "def"}   # => {:abc=>"def"}
h6 = {"abc" => "def"}  # => {"abc"=>"def"}

h1["abc"]  # => nil

h1.fetch("abc", "kitty")  # => "kitty"
h1.fetch("abc") { |x| x + "dbd" }  # => "abcdbd"
h1.fetch("abc") { "some_value" }  # => "some_value"
h.fetch("abc")  # ~> KeyError: key not found: "abc"

h = Hash.new    # => {}
h[:one] =  1    # => 1
h[:two] =  2    # => 2
h[:three] =  2  # => 2
h  # => {:one=>1, :two=>2, :three=>2}

h.each       # => #<Enumerator: {:one=>1, :two=>2, :three=>2}:each>
h.each { |key, value| p "key #{key} maps to #{value}" }  # => {:one=>1, :two=>2, :three=>2}
# Output
# >> "key one maps to 1"
# >> "key two maps to 2"
# >> "key three maps to 2"

h.each_pair  # => #<Enumerator: {:one=>1, :two=>2, :three=>2}:each_pair>
h.each_pair { |pair| p pair }  # => {:one=>1, :two=>2, :three=>2}
# Output:
# >> [:one, 1]
# >> [:two, 2]
# >> [:three, 2]

h.each_key    # => #<Enumerator: {:one=>1, :two=>2, :three=>2}:each_key>
h.each_value  # => #<Enumerator: {:one=>1, :two=>2, :three=>2}:each_value>
#Again these are more use with blocks. They return the keys and values separately

#Key? and include? are synonyms and examine the keys not the values
h.key? 1     # => false
h.key? :one  # => true
h.include? 1     # => false
h.include? :one  # => true

#Flatten returns the hash as an array like key1, value1, key2, value2
h.flatten             # => [:one, 1, :two, 2, :three, 2]

#Hash does the reverse - turns an array into a hash pairwise
array = ['a', 'b', 'c', 'd']  # => ["a", "b", "c", "d"]
Hash[*array]                  # => {"a"=>"b", "c"=>"d"}
#Note you need to "splat" the array. Can you think why?

h  # => {:one=>1, :two=>2, :three=>2}
other = { three: 3 }  # => {:three=>3}
h.merge other        # => {:one=>1, :two=>2, :three=>3}
h  # => {:one=>1, :two=>2, :three=>3}

h.delete(:one)  # => 1
h               # => {:two=>2, :three=>3}

h.delete_if { |k, v| v > 2 }     # => {:two=>2}
h                                # => {:two=>2}

h = { one: 1, two: 2, three: 3}  # => {:one=>1, :two=>2, :three=>3}

h.keep_if  { |k, v| v <= 2 }     # => {:one=>1, :two=>2}
h                                # => {:one=>1, :two=>2}

h[:one] = nil  # => nil
h              # => {:one=>nil, :two=>2}

h.delete(:one)  # => nil
h               # => {:two=>2}