calculator.rb

require "pry"

class Calculation
  attr_accessor :equation
  alias :eq :equation

  # @param [String] equation An input string containing a math equation.
  #   The only supported operations are addition, subtraction, multiplication, and division.
  #   The equation is assumed to be valid, with spaces between operators, and all operands being numbers.
  #
  # @example
  #   Calculation.new("1 * 5")
  #   Calculation.new("1 + -1")
  #   Calculation.new("1 + 4 - 2")
  #
  def initialize(equation)
    @equation = equation
  end

  OPERATORS = ["+", "-", "*", "/"]

  def operands
    eq.split(" ").delete_if{|str| OPERATORS.include?(str) }
  end #> ["1", "4", "2"]

  def operators
    eq.split(" ").keep_if{|str| OPERATORS.include?(str) }
  end #> ["+", "-"]

  def calculate(left_side, operator, right_side)
    left_side.send(operator, right_side)
  end

  def result
    operators.each_with_index do |operator, i|
      left_side = @memory || operands[i].to_f
      right_side = operands[i + 1].to_f
      @memory = calculate(left_side, operator, right_side)
    end

    return @memory
  end
end

equations = [
  "1 * 5",
  "1 + -1",
  "1 - 5",
  "10 / 3",
  #"1 + 1.1",
  #"1 - 1.1",
  #"1 * 1.1",
  #"1 / 1.1",
  "1 + 4 - 2"
]
equations.each do |eq|
  puts "#{eq} EQUALS #{Calculation.new(eq).result}"
end