derived_value.rb

# Derived value
# In Ruby, you can create derived values by defining methods that calculate a value based on the instance variables of an object.
# This allows you to create values that are not stored directly in the object but can be calculated when needed.

class Rectangle
  attr_reader :length, :width

  def initialize(length, width)
    @length = length
    @width = width
  end

  # derived value method to calculate the area of the rectangle based on the length and width instance variables.
  def area
    @length * @width
  end
end

rectangle = Rectangle.new(5, 3)
puts "Length: #{rectangle.length}"
puts "Width: #{rectangle.width}"
puts "Area: #{rectangle.area}"


# Other example using derived shopping cart total price based on the items in the cart.
class ShoppingCart
  attr_reader :items

  # empty array to hold the items in the cart
  def initialize
    @items = []
  end

  # method to add items to the cart
  def add_item(name, price)
    @items << { name: name, price: price}
  end

  # derived value method to calculate the total price of the items in the cart based on the price of each item.
  def total_price
    # reduce method to iterate through the items in the cart and calculate the total price by summing up the price of each item.
    # the initial value of total is set to 0 because if we did'nt set the initial value, the first item's price would be used as the initial value
    # wich means is incorrect because the first item's is a hash.
    @items.reduce(0) do |total, item|
      total + item[:price]
    end
  end
end

cart = ShoppingCart.new
cart.add_item("Book", 10)
cart.add_item("Pen", 2)
cart.add_item("Notebook", 5)

puts "Items in cart: #{cart.items.map { |item| item[:name] }.join(", ")}"
puts "Total price: $#{cart.total_price}"