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Scala:类型参数不符合trait Subtractable的类型参数界限

collections loops scala

Andrei Koch · 技术社区 · 7 年前

def group(string: String) = {
  val map = mutable.Map[Char, ListBuffer[Int]]()
  for (i <- string.indices) {
    val ch = string(i)
    if(map.contains(ch)) map(ch) += i
    else map += (ch -> ListBuffer(i))
  }
  map
}

当我试图编译时,出现了一个错误:

错误:(14,30)类型参数[?,Iterable[Any]与PartialFunction[Int with Char,Any]与scala.collection.generic类.Subtractable[\u>:Int with Char<:AnyVal,Iterable[Any]with PartialFunction[Int with Char,Any]withscala.collection.generic类.Subtractable[\u>:Int with Char<:AnyVal,Iterable[Any]with PartialFunction[Int with Char,任何]与scala.collection.generic类.Subtractable[\ugt;:Int with Char<:AnyVal,Equals]]{def seq:Iterable[Any]with PartialFunction[Int with Char,Any]}]{def seq:Iterable[Any]with PartialFunction[Int with Char,Any]{def seq:Iterable[Any]with PartialFunction[Int with Char,Any]}}]不符合trait Subtractable的类型参数界限[A,+Repr<:scala.collection.generic类.可减[A,Repr]] val v=对于(i<-字符串索引) {

似乎循环的值有问题。所以我在循环的最后一行添加了'true',现在一切正常:

def group(string: String) = {
  val map = mutable.Map[Char, ListBuffer[Int]]()
  for (i <- string.indices) {
    val ch = string(i)
    if(map.contains(ch)) map(ch) += i
    else map += (ch -> ListBuffer(i))
    true
  }
  map
}

Scala版本:2.12.6

3 回复 | 直到 7 年前

Aleksey Isachenkov 7 年前

正如你在书中看到的 https://docs.scala-lang.org/tutorials/FAQ/yield.html

所以,你的 for 环

for (i <- string.indices) {
  val ch = string(i)
  if(map.contains(ch)) map(ch) += i
  else map += (ch -> ListBuffer(i))
}

相当于

string.indices.foreach(i => {
  val ch = string(i)
  if(map.contains(ch)) map(ch) += i
  else map += (ch -> ListBuffer(i))
})

foreach

def foreach[U](f: A => U): Unit

map(ch) += i ListBuffer ,但是 map += (ch -> ListBuffer(i)) Map . 当编译器试图识别 U 在里面 foreach公司 f: Int => U ,它在 列表缓冲区 和 地图

你可以这样重写你的代码

def group(string: String) = {
    val map = mutable.Map[Char, ListBuffer[Int]]()

    def update(i: Int): Unit = {
        val ch = string(i)
        if(map.contains(ch)) map(ch) += i
        else map += (ch -> ListBuffer(i))
    }

    for (i <- string.indices) update(i)
    map
}

但最好使用标准方法

def group(string: String) = string.toCharArray.zipWithIndex.groupBy(_._1).mapValues(_.map(_._2))

Yuval Itzchakov 7 年前

方法不编译的原因是,返回的类型与 if-else 表情。一个回来了 ListBuffer[Int] Map[Char, ListBuffer[Int]] .

def group(string: String): mutable.Map[Char, ListBuffer[Int]] = {
  val map = mutable.Map[Char, ListBuffer[Int]]()
  for (i <- string.indices) {
    val ch = string(i)
    if (map.contains(ch)) {
      map(ch) += i
      map
    } else map += (ch -> ListBuffer(i))
  }
  map
}

没有可变参数的附加方法 Map ListBuffer 可以是:

def group(s: String): Map[Char, Int] = {
  s.split("\\W+")
   .flatten
   .zipWithIndex
   .groupBy { case (char, _) => char }
   .mapValues { arr => arr.map(_._2) }
}

lprakashv 7 年前

def group(string: String): mutable.Map[Char, List[Int]] = {

    val map = mutable.Map.empty[Char, List[Int]]

    string.zipWithIndex.foreach { case (char: Char, index: Int) =>
        map += (char -> (index :: map.get(char).getOrElse(List())))
    }

    map
}