calculator.rb

class Calculator

  def parse input
    input.split(" ")
  end

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

  def calculate input
    inp = parse input
    while inp.length > 1 do
      order_of_operations.each do | op |
      
        i = inp.find_index(op)

        if i 
          prev = inp[i-1].to_f
          nex =  inp[i+1].to_f
          result = operate(prev, op, nex)
          inp.slice!((i-1)..(i+1))
          inp[i-1] = result
        end
      end
    end
    inp[0]
  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)