Scala中的MapView(<未计算>) [英] MapView(<not computed>) in Scala

问题描述

我开始学习Scala，但发现有关 map 的一个令人困惑的问题.我的代码是这样的:

I started learning Scala but I found a confusing problem about map. My code is like this:

val mymap = Map(1 -> "james",2 -> "justin")
println(mymap.view.mapValues(x => "hi" + x))
println(mymap.view.mapValues(x => x.toUpperCase))

但结果是

MapView(<not computed>)
MapView(<not computed>)

我正在使用 view.mapValues ，因为不推荐使用 .map .我需要阅读任何建议或文档吗?

I am using view.mapValues because .map is deprecated. Any suggestion or doc I need to read about this?

推荐答案

尝试以下操作:

val mymap = Map(1 -> "james",2 -> "justin")
println(mymap.view.mapValues(x => "hi" + x).toMap)
println(mymap.view.mapValues(x => x.toUpperCase).toMap)

请注意，在Scala 2.12中，调用 mapValues 已弃用，并显示以下消息:

Note that in Scala 2.12 calling mapValues returned a Map. In Scala 2.13 mapValues was deprecated, with the message:

@deprecated("Use .view.mapValues(f). A future version will include a strict version of this method (for now, .view.mapValues(f).toMap).", "2.13.0")

为了获取 Map ，您应该调用 .view.mapValues(f).toMap .如果不调用 toMap ，则会得到 Stream vs Views vs Iterators .

In order to get a Map you should call .view.mapValues(f).toMap. If you don't call toMap, you get an instance of MapView, which is not materilized. For more information please read the great post: Stream vs Views vs Iterators.

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