scala - 如何在 Scala 中使用自动增量索引进行理解

Question

是否可以在 Scala 中使用自动增量计数器进行理解？

就像是

for (element <- elements; val counter = counter+1) yield NewElement(element, counter)

Answer 1

我相信，您正在寻找List和其他集合中zipWithIndex可用的方法。这是它的用法的小例子：

scala> val list = List("a", "b", "c")
list: List[java.lang.String] = List(a, b, c)

scala> list.zipWithIndex
res0: List[(java.lang.String, Int)] = List((a,0), (b,1), (c,2))

scala> list.zipWithIndex.map{case (elem, idx) => elem + " with index " + idx}
res1: List[java.lang.String] = List(a with index 0, b with index 1, c with index 2)

scala> for ((elem, idx) <- list.zipWithIndex) yield elem + " with index " + idx
res2: List[java.lang.String] = List(a with index 0, b with index 1, c with index 2)

Answer 2

for 理解不像 for 循环，因为这些术语是针对每个前一个术语进行评估的。例如，看看下面的结果。我认为这不是您要寻找的：

    scala> val elements = List("a", "b", "c", "d")
elements: List[java.lang.String] = List(a, b, c, d)

scala> for (e <- elements; i <- 0 until elements.length) yield (e, i)
res2: List[(java.lang.String, Int)] = List((a,0), (a,1), (a,2), (a,3), (b,0), (b,1), (b,2), (b,3), (c,0), (c,1), (c,2), (c,3), (d,0), (d,1), (d,2), (d,3))

Tenshi 的答案可能更符合您想要的结果，但我希望这个反例有用。