Ruby Method Explanation

Question

so I'm working through some examples of Ruby Methods from the rubymonk website and am having trouble interpreting what is going on in the code below. More specifically, I was hoping that someone might be able to help explain in layman's terms what each line in the code from the 'calculate' method is describing? I really appreciate any and all help!

  def add(*numbers)
     numbers.inject(0) { |sum, number| sum + number }  
  end

 def subtract(*numbers)
    sum = numbers.shift
    numbers.inject(sum) { |sum, number| sum - number }  
 end

 def calculate(*arguments)
   options = arguments[-1].is_a?(Hash) ? arguments.pop : {}
   options[:add] = true if options.empty?
   return add(*arguments) if options[:add]
   return subtract(*arguments) if options[:subtract]
 end

Answer 1

options = arguments[-1].is_a?(Hash) ? arguments.pop : {}

Create a new hash called options. This will either be assigned to the last element in the arguments array, or an empty one if that is not a hash. In ruby, like python, using -1 as an array index gets you the last element in an array.

options[:add] = true if options.empty?

Set the value in the hash that matches the key :add to true if the hash you just created is empty.

return add(*arguments) if options[:add]
return subtract(*arguments) if options[:subtract]

return the result of add or subtract with the same parameters you passed to this function, based on the state of the options hash you just created.

For example:

arguments = [{}, {:add => false, :subtract => true}]

would induce the subtract method if used as your parameter.