Scala机器模拟器

编程问答 2022-06-09

问题描述

我正在构建一个堆栈机模拟器，并且在弄清楚如何在Load和Store情况下更新环境/映射时遇到了麻烦。我的目标是弄清楚如何更新地图，并反复进行迭代以获取用于更新地图的正确值。

case class LoadI(s: String) extends StackMachineInstruction
case class  StoreI(s: String) extends StackMachineInstruction

    def emulateSingleInstruction(stack: List[Double],env: Map[String,Double],ins: StackMachineInstruction): (List[Double],Map[String,Double]) = {
        ins match{
            case AddI => stack match{
                case i1 :: i2 :: tail => (i1 + i2 :: tail,env)
                case _ => throw new IllegalArgumentException()
            }
            case PushI(f) => (f :: stack,env)

            //broken
            case LoadI(s) => stack match {
                case Nil => throw new IllegalArgumentException()
                case i1 :: tail => (tail,env) match {
                    case (top,env) => (top,env + s -> top ) // Not clear on how to update an environment
                }
            }
            //broken
            case StoreI(s) => stack match {
                case Nil => throw new IllegalArgumentException()
                case i1 :: tail => // Need to take the value that s maps to in the environment. Let the value be v. Push v onto the top of the stack.
            }

            case PopI => stack match{
                case Nil => throw new IllegalArgumentException()
                case i1 :: tail => {
                    (tail,env)
                }
            }
        }
    }

这是我在另一个文件中拥有一个测试用例的示例

test("stack machine test 3") {
        val lst1 = List(PushI(3.5),PushI(2.5),PushI(4.5),PushI(5.2),AddI,LoadI("x"),LoadI("y"),LoadI("z"),StoreI("y"),LoadI("w"))
        val fenv = StackMachineEmulator.emulateStackMachine(lst1)
        assert(fenv contains "x")
        assert(fenv contains "y")
        assert(fenv contains "z")
        assert( math.abs(fenv("x") - 9.7 ) <= 1e-05 )
        assert( math.abs(fenv("y") - 2.5 ) <= 1e-05 )
        assert( math.abs(fenv("z") - 3.5 ) <= 1e-05 )
    }

解决方法

您可以使用.updated更新地图。或添加一个元组，但是Scala必须知道某物是一个元组（map + a -> b被视为(map + a) -> b，因此您必须使用map + (a -> b)）。

您可以使用.get，。getOrElse或.apply从地图中提取值。它们将：返回Option，缺少则返回值或默认值，如果缺少则提取值或抛出。所以最后一种需要检查。

如果您还记得模式可以通过模式匹配来组合，则可以将解释器编写为：

def emulateSingleInstruction(stack: List[Double],env: Map[String,Double],ins: StackMachineInstruction): (List[Double],Map[String,Double]) =
  (ins,stack) match {
    case (AddI,i1 :: i2 :: tail)          => ((i1 + i2) :: tail) -> env
    case (PushI(f),_)                     => (f :: stack) -> env
    case (LoadI(s),i1 :: tail)            => tail -> env.updated(s,i1)
    case (StoreI(s),_) if env.contains(s) => (env(s) :: stack) -> env
    case (PopI,_ :: tail)                 => tail -> env
    case _                                 => throw new IllegalArgumentException()
  }
emulationloadloadloadpattern-matchingscalascalastorestorestore