Scala 模式匹配与元组

Question

我正在处理 Coursera 课程的第一个作业。

对于以下代码，我遇到了编译时错误。

    object pascal {
        def main(c: Int, r: Int) = {
                pascal(c, r)
            }
            def pascal(c: Int, r: Int) = {
                (c, r) match {
                    case ((r < 0) || (c < 0)) => throw new Exception
                                                 ("r and c must be > 0")
                    case r == 0 => 1
                    case r == 1 => 1
                    case c == 0 => 1
                    case r == c => 1
                    case _ => pascal(r-1,c-1) + pascal(r-1,c-1)
            }
        }
     }

vagrant@precise64:/vagrant/Workspace/Scala/hw1$ scalac pascal.scala
pascal.scala:7: error: not found: value ||
                        case ((r < 0) || (c < 0)) => throw ...
                                      ^
pascal.scala:8: error: value == is not a case class constructor, nor 
does it have an unapply/unapplySeq method
                        case r == 0 => 1

请指教。

谢谢。

Answer 1

这是 scala 中的无效语法：您需要使用所谓的 guards 来检查布尔属性：

def pascal(c: Int, r: Int) = {
   (c, r) match {
     case (c,r) if ((r < 0) || (c < 0)) => throw new Exception
                                                 ("r and c must be > 0")
     ...
}

顺便说一句，这样写代码更习惯：

def pascal(c: Int, r: Int) = {
   require((r >= 0) && (c >= 0), "r and c must be >= 0")
   (c, r) match {
     case (_, 1) => 1
     ...
}

您必须将错误消息更改为“r 和 c 必须 >= 0”