我正在学习Scala,我在编写简单的reducer函数时遇到了一些麻烦。我想计算一个值出现在列表中的次数。因此,我编写了一个reducer / folding函数,它将一个不可变映射作为初始值,然后迭代列表"更新"地图,其中包含值在列表中显示的次数。非常简单的东西,但我遇到了类型系统的问题(啊......在JavaScript和Python上花了太多时间)
这是功能:
lapply(1 : 100, function (i) { HoltWinters(mydata[, group == i],
beta = FALSE, gamma = FALSE) } )
我无法理解为什么会出现以下类型错误:
def times(chars: List[Char]): List[(Char, Int)] = {
val acc:Map[Char, Int] = Map()
def count(acc:Map[Char, Int], c:Char) = {
if (acc contains c) acc + (c -> (acc(c) + 1))
else acc + (c -> 1)
}
chars.fold(acc)(count).toList
}
[error] /Users/giuseppe/courses/scala_functional_programing/lessons/week_4/assignment/src/main/scala/patmat/Huffman.scala:85: type mismatch;
[error] found : (Map[Char,Int], Char) => scala.collection.immutable.Map[Char,Int]
[error] required: (Any, Any) => Any
[error] chars.fold(acc)(count)
[error] ^
[error] /Users/giuseppe/courses/scala_functional_programing/lessons/week_4/assignment/src/main/scala/patmat/Huffman.scala:85: type mismatch;
[error] found : Any
[error] required: List[(Char, Int)]
[error] chars.fold(acc)(count)
[error] ^
[error] two errors found
的子类型是Map[Char, Int]
吗?
*编辑*
更有趣的是,我刚刚发现,如果我使用Any
,它就能正常运行。 :confused:为什么??
任何帮助都会非常感激。谢谢
答案 0 :(得分:2)
看起来你需要foldLeft功能而不是折叠。 它有适当的签名。
def foldLeft[B](z: B)(f: (B, A) => B): B = { // in your case B is Map and A is Char
def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1 = foldLeft(z)(op) // in your case arguments of count functions should be same