What is the difference between the map
and flatMap
functions of Iterable
?
views:
1394answers:
5From scaladoc:
- map
Returns the iterable resulting from applying the given function f to each element of this iterable.
- flatMap
Applies the given function f to each element of this iterable, then concatenates the results.
Look here: 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).
Here is a pretty good explanation:
http://www.codecommit.com/blog/scala/scala-collections-for-the-easily-bored-part-2
Using list as an example:
Map's signature is:
map [B](f : (A) => B) : List[B]
and flatMap's is
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]
This will also give you an idea that flatmap will "flatten" lists.
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)
The above is all true, but there is one more thing that is handy: flatMap
turns a List[Option[A]]
into List[A]
, with any Option
that drills down to None
, removed. This is a key conceptual breakthrough for getting beyond using null
.
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:
for {line <- lines
word <- line split "\\W+"}
yield word.length
this translates into:
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.