Ruby Day 4 Lecture Notes - Hash & Symbols

1-array-display-students-age.rb

#                  0        1        2        3
students =     [ "Peter", "Mary", "George", "Emma" ]
student_ages = [ 24     , 25    , 22      ,  20    ]

# Write a program that prints this:
# Peter is 24 years old

students.each_with_index do |student, index|
  age = student_ages[index]
  p "#{student} is #{age} years old"
end

2-hash-crud.rb

# CRUD

# Array CRUD
# cities = ["Paris", "London"]
# C: cities << "Rio de Janeiro"
# R: cities[2]
# U: cities[2] = "Melbourne"
# D: cities.delete_at(2) / cities.delete("London")

# Hash CRUD
paris = {
  "country" => "France",
  "population" => 2211000
}
# C:
paris["monument"] = "Pain au Chocolat"
p paris["monument"]
# R:
p paris["country"]
# U:
paris["country"] = "Moon"
p paris["country"]
# D:
paris.delete("country")
p paris

3-hash-each.rb

paris = {
  "country" => "France",
  "population" => 2211000
}

# Let's output this using Hash#each:
# The key X stores the value Y
paris.each do |key, value|
  p "The key #{key} stores the value #{value}"
end

# BE CAREFUL!!! If you don't use both parameters on the
# block you'll get an array as the iterated object.
paris.each do |object|
  p object # => ["country", "France"]
end

4-1-hash-to-symbol.rb

# Our old hash:
paris = {
  "country" => "France",
  "population" => 2211000
}

# Old Ruby syntax:
paris = {
  :country => "France",
  :population => 2211000
}

# New Ruby syntax:
paris = {
  country: "France",
  population: 2211000
}

4-hash-custom-methods.rb

paris = {
  "country" => "France",
  "population" => 2211000,
  "star_monument" => "Tour Eiffel"
}

# Checking keys:
p paris.has_key?("country")
p paris.has_key?("language")

# Print the keys of this hash:
p paris.keys

# Print the values of this hash:
p paris.values

5-hash-last-argument.rb

# Rebuilding the tag method of yesterday
# We're cheating on the blocks. Sorry!
def tag(tag_name, attributes = {})
  attributes_string = attributes.map { |name, value| "#{name}='#{value}'" }.join(" ")
  "<#{tag_name} #{attributes_string}>#{yield}</#{tag_name}>"
end

my_tag = tag("a", href: "http://lewagon.org", class: "btn") do
  tag("h1") do
    "Hello"
  end
end

p my_tag