Scala Map.mapValues stackoverflowerror [英] Scala Map.mapValues stackoverflowerror
问题描述
以下代码导致最后一行出现 StackOverflowError.
The following code results in a StackOverflowError on the last line.
object StackTest extends App{
@tailrec def incrementValues(acc: Map[String, Int], inc: Int): Map[String, Int] = {
if(inc == 0) acc
else incrementValues(acc.mapValues(_ + 1), inc - 1)
}
val myMap = incrementValues(Map("key" -> 0), 10000)
myMap.foreach(println)
}
在 Scala 2.11.2 中:
In Scala 2.11.2:
Exception in thread "main" java.lang.StackOverflowError
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:777)
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:777)
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
...
查看 MapLike 的源代码,我可以看到它使用的是 MappedValues 对象,这看起来像一个视图:
Looking at the source for MapLike I can see it's using a MappedValues object, which seems like a view:
protected class MappedValues[C](f: B => C) extends AbstractMap[A, C] with DefaultMap[A, C] {
override def foreach[D](g: ((A, C)) => D): Unit = for ((k, v) <- self) g((k, f(v)))
def iterator = for ((k, v) <- self.iterator) yield (k, f(v))
override def size = self.size
override def contains(key: A) = self.contains(key)
def get(key: A) = self.get(key).map(f)
}
/** Transforms this map by applying a function to every retrieved value.
* @param f the function used to transform values of this map.
* @return a map view which maps every key of this map
* to `f(this(key))`. The resulting map wraps the original map without copying any elements.
*/
def mapValues[C](f: B => C): Map[A, C] = new MappedValues(f)
使它实际映射值的最佳方法是什么?
What's the best way to make it actually map the values?
推荐答案
mapValues
被认为是一个陷阱,因为它确实只创建了一个包装视图函数,而不是急切地生成一个新集合.因此,在您的示例中,您创建了一个嵌套级别为 10,000 的数据结构.
mapValues
is known to be a trap, as it indeed creates only a wrapping view function instead of eagerly producing a new collection. Therefore, in your example you create a data structure of nesting level 10,000.
您可以使用常规的 map
方法:
You can use the regular map
method:
acc.map(tup => (tup._1, tup._2 + 1))
或
acc.map { case (key, value) => (key, value + 1) }
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