Introducing Hashes

Warmup

Ruby Doc defines a hash as “a dictionary-like collection of unique keys and their values. Also called associative arrays, they are similar to Arrays, but where an Array uses integers as its index, a Hash allows you to use any object type.”

• What information can you tease from this definition?
• What else do you know about hashes?

Dictionary-Like/Keys & Values

• Hashes allow us to store/look up values by keys of our choosing.
• In a dictionary the keys would be words and the values definitions.
• A hash would allow us to store all of those words out of order and still be able to find their definitions.

Key Value Pairs

• Collection of key/value pairs
• Key must be unique
• Values can be any object (including other arrays and hashes)

Syntax

new_hash = {}
# Different syntax
new_hash = Hash.new
# Different syntax with default value
new_hash = Hash.new(0)

Key Syntax: Hash Rockets

old_tv = {
  "screen size" => 50,
  "price" => 300,
  "brand" => "Samsung"
}

Key Syntax: Symbols

new_tv = {
  screen_size: 50,
  price: 300,
  brand: "Samsung"
}

Advantages of Using Symbols

• Strings are compared character by character
• Symbols are compared by their object_id
• Symbols help your code run faster

Accessing Attributes by Keys

• Symbols by themselves (outside a key/value pair) look like this: :brand

old_tv.keys
old_tv.values
old_tv["screen size"]
old_tv["brand"]

new_tv.keys
new_tv[:screen_size]
new_tv[:price]

Adding Key/Value Pairs

new_tv[:resolution] = "720p"
new_tv

Pair Exercise

• See instructions in lesson plan

Wrap Up

• What is a symbol? How is it different than a String?
• What is the advantage of using a String? What is the advantage of using a Symbol? Which is better for Hashes?
• What is different about using symbols in Hashes?
• Describe some useful Hash methods. Where can you look to find more Hash methods?