模式匹配问题,在Scala中实现SplitAt [英] Problems with Pattern matching, implementing SplitAt in scala
问题描述
我正在尝试使用模式匹配来实现scala splitAt,这就是我正在尝试做的事情:
I am trying to implement the scala splitAt using pattern matching and this is what I am trying to do:
def split[T](someIndex:Int,someList:List[T]):(List[T],List[T]) = {
def splitHelper[T](currentIndex:Int,someList:List[T],headList:List[T]):(List[T],List[T])= {
(currentIndex,someList) match {
case (someIndex,x::tail) => (x::headList,tail)
case (currentIndex,x::y) => splitHelper(currentIndex+1,y,x::headList)
case _ => (headList,headList)
}
}
splitHelper(0,someList,List[T]())
}
编译器抱怨说:
<console>:15: error: unreachable code
case (currentIndex,x::y) => splitHelper(currentIndex+1,y,x::headList)
有人可以指出我在这里做错了什么,为什么我遇到无法到达的代码错误.
Can someone point out what I am doing wrong here and why I am getting the unreachable code error.
谢谢
推荐答案
在模式匹配中应使用`someIndex`和`currentIndex`(常量).
You should use `someIndex` and `currentIndex` (constants) in pattern matching.
scala> val a = 1
a: Int = 1
scala> 2 match {
| case a => println(a)
| }
2
scala> 2 match {
| case `a` => println("a")
| case _ => println("Oops")
| }
Oops
Scala编程第15章,第一版. 案例类和模式匹配" 由Martin Odersky,Lex Spoon和Bill Venners撰写:
如果需要,您仍然可以为模式使用小写名称 常量,使用两个技巧之一.首先,如果常量是一个字段 对于某些对象,您可以在其前面加上限定符.例如pi 是一个可变模式,但是this.pi或obj.pi是常量,即使 它们以小写字母开头.如果这样不起作用(因为pi 是局部变量,例如),您也可以将变量括起来 姓名记在后面.
If you need to, you can still use a lowercase name for a pattern constant, using one of two tricks. First, if the constant is a field of some object, you can prefix it with a qualifier. For instance, pi is a variable pattern, but this.pi or obj.pi are constants even though they start with lowercase letters. If that does not work (because pi is a local variable, say), you can alternatively enclose the variable name in back ticks.
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