Indexing and Appending

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 39 of 57.

obj[key] calls the method [], and obj[key] = value calls []=. Defining them gives a class the square-bracket syntax of arrays and hashes, with whatever meaning suits it:

class Inventory
  def initialize
    @stock = Hash.new(0)
  end

  def [](name)
    @stock[name]
  end

  def []=(name, qty)
    @stock[name] = qty
  end
end

inv = Inventory.new
inv["apple"] = 5
inv["apple"] += 2
puts inv["apple"]
puts inv["pear"]

Output:

7
0

[] can take several arguments, for example a row and a column, and it can check them. IndexError is the usual error for a position that does not exist:

class Board
  def initialize(size)
    @size = size
    @cells = Array.new(size) { Array.new(size, ".") }
  end

  def [](row, col)
    check(row, col)
    @cells[row][col]
  end

  def []=(row, col, mark)
    check(row, col)
    @cells[row][col] = mark
  end

  def check(row, col)
    raise IndexError, "no cell #{row},#{col}" unless row.between?(0, @size - 1) && col.between?(0, @size - 1)
  end
end

b = Board.new(3)
b[1, 1] = "X"
puts b[1, 1] + b[0, 0]
begin
  b[3, 0]
rescue IndexError => e
  puts e.message
end

Output:

X.
no cell 3,0

<< is the append operator. Returning self from it allows chaining, like list << a << b on arrays:

class Shelf
  def initialize
    @books = []
  end

  def <<(title)
    @books << title
    self
  end

  def to_s
    @books.join(" | ")
  end
end

shelf = Shelf.new
shelf << "Dune" << "Emma" << "Ulysses"
puts shelf

Output:

Dune | Emma | Ulysses

An assignment through []= returns the assigned value, not what the method returns, as with any setter. Brackets and << give a class a small, familiar interface:

inv["apple"] = 5          # calls []=("apple", 5), the expression is 5
inv["apple"]              # calls []("apple")
shelf << "Dune" << "Emma"  # << returns self, so it chains
board[1, 2] = "X"         # []=(1, 2, "X")

Challenge

Easy

Complete the class Inventory. inv[name] returns the quantity of an item (0 when unknown), inv[name] = qty sets it, and inv << name adds one of that item and returns the inventory so calls can be chained. to_s returns apple: 3, pear: 1 in alphabetical order of the names, leaving out items with quantity 0, or empty.

The supplied code reads commands: set apple 5, add apple pear pear (which chains << for every name), get apple and print, and prints the result of get and print.

Your code goes in inventory.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'inventory'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
inv = Inventory.new
input.each do |line|
  cmd, *args = line.split
  case cmd
  when "set" then inv[args[0]] = args[1].to_i
  when "add" then args.reduce(inv) { |target, name| target << name }
  when "get" then puts inv[args[0]]
  else puts inv
  end
end