Simple calculator (no parentheses support)

class Calculator
  def tokenize(input)
    input.split.map do |token|
      Float(token) rescue token
    end
  end

  # operation(3.0, "+", 4.0) returns 7.0 etc.
  def operation(a, op, b)
    raise "Expected A and B to be numbers" unless a.is_a?(Float) and b.is_a?(Float)
    case op
    when "+"
      a + b
    when "-"
      a - b
    when "*"
      a * b
    when "/"
      a / b
    else
      raise "Unknown operator #{op}"
    end
  end

  # turn [..., 3, "+", 4, ...]
  # into [..., 7, ...]
  # where index_op points at operator
  def replace_expr_with_result(tokens, index_op)
    a, op, b = tokens[index_op-1..index_op+1]
    result = operation(a, op, b)
    tokens[index_op-1..index_op+1] = [result]
  end

  def calculate(input)
    tokens = tokenize(input)

    # First do all * and /
    loop do
      index_op = tokens.index do |token|
        token == "*" or token == "/"
      end
      break unless index_op
      replace_expr_with_result(tokens, index_op)
    end

    # Then do all + and -
    loop do
      index_op = tokens.index do |token|
        token == "+" or token == "-"
      end
      break unless index_op
      replace_expr_with_result(tokens, index_op)
    end

    raise "Extra stuff after all operations: #{tokens.inspect}" if tokens.size > 1

    tokens[0]
  end
end

input = "2 + 3 * 4 - 8 / 4 * 3 - 6 - 8"
result = Calculator.new.calculate(input)
expected = eval(input)

p [result, expected]