W1D3 Lecture Notes - Arrays, Hashes, Scope

lectures_notes.md

Data Structures

Arrays

A way of storing lists in Ruby

names = ["Faisal", "Sara", "Jane"]

An array can store a mix of data types.

arr = [[1, 2], "Yo", 1, 21, true, [1, 2, 3]]

puts arr[0][0] # outputs: 1

Real-life Analogy: Tennis balls pack

Hashes

A way of storing key-value pairs

person = {name: "Faisal", age: 24, "hobbies": ["Tennis", "Soccer"]}

Real-life Analogy: Drawers with labels

old syntax: person = {:name => "Faisal", :age => 24}

Hash keys must be either Symbols or Strings. Values can be anything (Strings, Arrays, Hashes)

Nesting hashes inside arrays

List of people, with each person containing attributes about them

people = [
 {name: "Faisal", age: 24, "hobbies": ["Tennis", "Soccer"]},
 {name: "Sara", age: 29, "hobbies": ["Basket Ball"]}
]

Nest arrays inside arrays

X, Y points in a graph

data_points = [[2, 1], [3, 2], [4, 1]]

Nest hashes inside hashes

Keep track of a person (name, number) AND their home and mobile numbers.

person = {
  name: "Faisal",
  city: "Toronto", 
  phone_numbers: {mobile: "6479978509", home: "2892324545"}
}

Working with Arrays and Hashes

Let's say we have the following array

people = [
 {name: "Faisal", age: 24, "hobbies": ["Tennis", "Soccer"]},
 {name: "Sara", age: 29, "hobbies": ["Basket Ball"]}
]

Get the name of the first friend

people[1][:name] # outputs: Sara

Retrieve the names of all people in an Array

people_names = []
people.each do |person|
  people_names = person[:name]
end

You can accomplish the same using a fancier method called #map

people_names = people.map {|person| person[:name]}

Symbols vs Strings

Symbols and Strings store the same type of data (lists of characters, a.k.a words). Hash keys can be either Strings or Symbols.

Symbols are a more efficient way of storing a string. You can read more about the details here: http://www.reactive.io/tips/2009/01/11/the-difference-between-ruby-symbols-and-strings/

Documentation for Arrays and Hashes

Ruby Docs: http://ruby-doc.org/

Use the search bar.

Take a look at docs for Enumerable, Array, Hash.

Variable Scope

Use instance variables to share variables accross different methods. Instance variables start with a @ character.

# File called friends.rb
require_relative('./friends')
# above line is the equivalent of copy/pasting the code in a file called friends.rb into the current file

def friend_names
  friend_names = []
  # friends is out of scope if it is not defined as an instance variable
  @friends.each do |friend|
    friend_names << friend[:name]
  end
  # return a list of friends' names
  friend_names
end

def show_friend_names
  puts "The friend names are"
  puts friend_names
end

show_friend_names

# friends.rb file
@friends = [
  {name: "Faisal", age: 24, "hobbies": ["Tennis", "Soccer"]},
  {name: "Sara", age: 29, "hobbies": ["Basket Ball"]}
]