Hash

a hash is an abstract data type

• unordered collection of key/value pairs
• keys must be unique
• values can be anything

(image credit: Jeffrey W, CC-BY )

Hash is also known as...

• Map
• Associative Array
• Dictionary
• Name/Value Pair Store
• Key/Value Pair Store

Setting and Getting Hash Values

states = Hash.new

states["CA"] = "California"
states["MA"] = "Massachusetts"
states["NY"] = "New York"

states["MA"].reverse #=> "sttesuhcassaM"

Hash literals

a Hash can be defined literally (inline) with braces e.g.

states = {
           "CA" => "California",
           "MA" => "Massachusetts",
           "NY" => "New York"
         }

states["MA"] #=> "Massachusetts"

The little => arrow is called a hash rocket.

Remember, strings are not symbols!

states = {:MA => "Massachusets"}
states["MA"] #=> nil
states[:MA]  #=> "Massachusets"

This is such an annoying problem that Rails invented a new version of Hash named HashWithIndifferentAccess that converts string keys into symbol keys.

(HashWithIndifferentAccess is such an annoying name that many people call it Mash instead.)

Hash Literals with Symbol Keys

Ruby 1.8 or 1.9:

{:foo => "bar", :baz => "baf"}

Ruby 1.9 only:

{foo: "bar", baz: "baf"}

Hash arguments

When passing a hash to a method,

the braces are optional...

...**if** you're calling a method

...**and** the hash is the final argument

invite({:name => "Alice", :age => 18})

invite :name => "Alice", :age => 18

This is called "the default hash" (which is a silly name for it).

hash parameters plus hash literals => named parameters

These are all equivalent:

User.new({:name => "Alex", :email => "alex@stinky.com"})

User.new(:name => "Alex", :email => "alex@stinky.com")

User.new :name => "Alex", :email => "alex@stinky.com"

User.new name: "Alex", email: "alex@stinky.com"

User.new email:"alex@stinky.com", name:"Alex"

...so passing a hash literal kinda sorta almost looks like named parameters.

(Ruby 2.0 will support proper named parameters, using the same calling syntax.)

Hash methods

• each, each_pair
• keys, values
• has_key?, has_value?
• merge, merge!
• delete, delete_if

merge to combine two hashes

Here's a handy trick:

class Hash
  alias_method :<<, :merge!
  alias_method :+, :merge
end

{foo: 1} << {bar: 2}
=> {:foo=>1, :bar=>2}   # destructive

{foo: 1} + {bar: 2}
=> {:foo=>1, :bar=>2}   # constructive

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