Scala 中的 map 和 foreach 方法之间的区别? [英] Difference between map and foreach method in Scala?

问题描述

当我开始编程 Scala 时，我确实有一个很大的疑问.我想知道 Scala 中的 map 方法是如何工作的.是顺序处理还是多线程处理?更重要的是，我想知道为什么 map 方法比 while 或 foreach 更快?

I do have one big doubt when I started programming Scala. I want to know how the map method in scala works. Whether it's processing sequentially or in multithreaded? And more importantly I would like to know that why map method is faster than while or foreach?

val list = List(1,2,3,45,12)
list.map(x => x)
list.foreach(x => println(x))

推荐答案

首先，这两种操作有无限的不同.map 是给定函数 A => 的列表的转换.B，而 foreach 产生 Unit，通常用于副作用.

Firstly, the two operations are infinitely different. map is a transformation of the list given a function A => B, whereas foreach yields Unit and is usually used for side-effects.

我猜测 foreach 与 map 相比，在执行所需的周期方面更快"，后者创建了一个新集合(在这种情况下)作为结果功能.但比较这两者其实就是比较苹果和橙子.

I would guess that foreach is "faster" in terms of cycles needed to execute compared to map which creates a new collection (in this case) as the result of the function. But comparing these two is really comparing apples and oranges.

map 只有在调用它的集合是并行集合时才会是并行的.所以在你的例子中:

map will only be parallel if the collection on which it is invoked is a parallel collection. So in your example:

list.map(x => x)

不是并行的而是顺序的，但是

is not parallel and is sequential, but

list.par.map(x => x)

应该是平行的.显然，这种用法需要考虑各种注意事项.同一个并行集合也有一个 foreach 方法.

would be parallel. There are obviously various caveats that need to be taken into account with this usage. The same parallel collection has a foreach method as well.

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