Scala Iterable#map vs. Iterable#flatMap
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scala Iterable#map vs. Iterable#flatMap
提问by Landon Kuhn
What is the difference between the mapand flatMapfunctions of Iterable?
map和 的flatMap功能有Iterable什么区别？
采纳答案by agilefall
Here is a pretty good explanation:
这是一个很好的解释：
http://www.codecommit.com/blog/scala/scala-collections-for-the-easily-bored-part-2
http://www.codecommit.com/blog/scala/scala-collections-for-the-easily-bored-part-2
Using list as an example:
以列表为例：
Map's signature is:
地图的签名是：
map [B](f : (A) => B) : List[B]
and flatMap's is
和 flatMap 是
flatMap [B](f : (A) => Iterable[B]) : List[B]
So flatMap takes a type [A] and returns an iterable type [B] and map takes a type [A] and returns a type [B]
所以 flatMap 接受一个类型 [A] 并返回一个可迭代类型 [B] 并且 map 接受一个类型 [A] 并返回一个类型 [B]
This will also give you an idea that flatmap will "flatten" lists.
这也会让您知道 flatmap 将“展平”列表。
val l = List(List(1,2,3), List(2,3,4))
println(l.map(_.toString)) // changes type from list to string
// prints List(List(1, 2, 3), List(2, 3, 4))
println(l.flatMap(x => x)) // "changes" type list to iterable
// prints List(1, 2, 3, 2, 3, 4)
回答by kikibobo
The above is all true, but there is one more thing that is handy: flatMapturns a List[Option[A]]into List[A], with any Optionthat drills down to None, removed. This is a key conceptual breakthrough for getting beyond using null.
以上都是正确的，但还有一件很方便的事情： flatMap将 aList[Option[A]]变成List[A]，并删除任何Option向下钻取到 的None。这是超越使用null.
回答by Daniel C. Sobral
lines.map(line => line split "\W+") // will return a list of arrays of words
lines.flatMap(line => line split "\W+") // will return a list of words
You can see this better in for comprehensions:
您可以更好地理解这一点：
for {line <- lines
word <- line split "\W+"}
yield word.length
this translates into:
这转化为：
lines.flatMap(line => line.split("\W+").map(word => word.length))
Each iterator inside for will be translated into a "flatMap", except the last one, which gets translated into a "map". This way, instead of returning nested collections (a list of an array of a buffer of blah, blah, blah), you return a flat collection. A collection formed by the elements being yield'ed -- a list of Integers, in this case.
for 中的每个迭代器都将被转换为“flatMap”，除了最后一个被转换为“map”。这样，您将返回一个平面集合，而不是返回嵌套集合（一个包含 blah、blah、blah 缓冲区数组的列表）。由被产生的元素形成的集合——在这种情况下是整数列表。
回答by skaffman
回答by skaffman
Look here: http://www.codecommit.com/blog/scala/scala-collections-for-the-easily-bored-part-2
看这里：http: //www.codecommit.com/blog/scala/scala-collections-for-the-easily-bored-part-2
"Search for flatMap" - there is a really good explanation of it there. (Basically it is a combination of "flatten" and "map" -- features from other languages).
“搜索 flatMap” - 那里有一个很好的解释。（基本上它是“扁平化”和“映射”的组合——来自其他语言的特征）。
