calculator_recursive.rb

class Calculator

  def parse input
    input.split(" ")
  end

  def order_of_operations
    ["*", "/", "+", "-"]
  end

  def calculate input
    recursive_calc(parse input)
  end

  def recursive_calc(input)
    return input[0] if input.length == 1 

    order_of_operations.each do | op |
      i = input.find_index(op)

      if i 
        prev = input[i-1].to_f
        nex =  input[i+1].to_f
        result = operate(prev, op, nex)
        input.slice!((i-1)..(i+1))
        input[i-1] = result
        return recursive_calc(input)
      end
    end
  end

  def operate(prev, op, nex)
    case op
    when "*"
      prev * nex
    when "/"
      prev / nex
    when "+"
      prev + nex
    when "-"
      prev - nex
    else
      raise
    end
  end

end

input = '7 - 2 + 3 / 4 * 5'
expected = (7 - (2 + (3 / (4 * 5.to_f))))
actual = Calculator.new.calculate(input)
puts actual
puts (expected == actual)