Scala:为什么 mapValues 会生成一个视图，是否有任何稳定的替代方案?

问题描述

刚才我惊讶地发现 mapValues 生成了一个视图.结果如下例所示:

Just now I am surprised to learn that mapValues produces a view. The consequence is shown in the following example:

case class thing(id: Int)
val rand = new java.util.Random
val distribution = Map(thing(0) -> 0.5, thing(1) -> 0.5)
val perturbed = distribution mapValues { _ + 0.1 * rand.nextGaussian }
val sumProbs = perturbed.map{_._2}.sum
val newDistribution = perturbed mapValues { _ / sumProbs }

这个想法是我有一个分布，它受到一些随机性的干扰，然后我将它重新归一化.代码实际上违背了它的初衷:由于 mapValues 产生了一个 view，_ + 0.1 * rand.nextGaussian 总是重新计算 _ + 0.1 * rand.nextGaussian使用代码>扰动.

The idea is that I have a distribution, which is perturbed with some randomness then I renormalize it. The code actually fails in its original intention: since mapValues produces a view, _ + 0.1 * rand.nextGaussian is always re-evaluated whenever perturbed is used.

我现在正在做类似 distribution map { case (s, p) =>(s, p + 0.1 * rand.nextGaussian) }，但这只是有点冗长.所以这个问题的目的是:

I am now doing something like distribution map { case (s, p) => (s, p + 0.1 * rand.nextGaussian) }, but that's just a little bit verbose. So the purpose of this question is:

1. 提醒不知道这一事实的人.
2. 寻找他们使 mapValues 输出 views 的原因.
3. 是否有替代方法可以生成具体的Map.
4. 有没有其他常用的收集方法有这个陷阱.

1. Remind people who are unaware of this fact.
2. Look for reasons why they make mapValues output views.
3. Whether there is an alternative method that produces concrete Map.
4. Are there any other commonly-used collection methods that have this trap.

谢谢.

推荐答案

有一张关于这个的票，SI-4776(由 YT 提供).

There's a ticket about this, SI-4776 (by YT).

介绍它的提交是这样说的:

The commit that introduces it has this to say:

根据 jrudolph 的建议，制作了 filterKeys 和 mapValues转换抽象映射和不可变的重复功能地图.将 transform 和 filterNot 从不可变映射到通用映射.由 phaler 审核.

我无法找到 jrudolph 的原始建议，但我认为这样做是为了使 mapValues 更有效率.提出这个问题，这可能会让人感到意外，但 mapValues 如果您不太可能对这些值进行多次迭代，则效率会更高.

I have not been able to find the original suggestion by jrudolph, but I assume it was done to make mapValues more efficient. Give the question, that may come as a surprise, but mapValues is more efficient if you are not likely to iterate over the values more than once.

作为一种变通方法，可以执行 mapValues(...).view.force 来生成新的 Map.

As a work-around, one can do mapValues(...).view.force to produce a new Map.

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