Scala-为什么::不更改列表? [英] Scala - why does :: not change a List?

问题描述

我刚刚编写了一些有趣的代码，但我有一个疑问，为什么它不起作用?

I have just written this code for some fun and I have a question why it isn't working out?

  val list = List[Int]()

  while (list.length < 20) {  
    Random.nextInt(100) :: list
  }

  println(list)
}

似乎没有任何内容写入列表，但是为什么会这样呢?我必须使其可变吗?为什么::运算符在这里不能正常工作?

Seems like nothing is written to the list, but why is that so? Do I have to make it mutable? And why is here the :: operator not working properly?

推荐答案

因为x :: xs返回一个新列表，其中第一个元素是x，后跟xs.

Because x :: xs returns a new list, where the first element is x, followed by xs.

因此，要使您的代码起作用，请将列表设为var，然后在循环内重新分配它:

So, to make your code work, make list a var and re-assign it inside the loop:

var list = List[Int]()

while(list.length < 20)
  list = Random.nextInt(100) :: list

但是，在Scala中这样做的惯用方式是根本不使用突变(尽管有时局部可变状态是可以的).通常，"While"循环可以用递归函数代替:

However, the idiomatic way of doing this in Scala is to not use mutations at all (local mutable state is fine sometimes though). "While" loops can usually be replaced with a recursive function:

def go(xs: List[Int]) =
  if (xs.length >= 20) xs
  else go(Random.nextInt(100) :: xs)

go(List())

也可以使用

This particular scenario can also be solved using List.fill instead

val list = List.fill(20)(Random.nextInt(100))

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