35th: List of map, FlatMap, foreach, filter Operation code Combat

The role of the list's map function:

The map parameter is a function, and each element in the list is applied to the function, and a new collection is returned.

For example we have a list (1,2,3,4) and we need to build list (2,4,6,8)

Then we can use the map function to pass in a * * function

As follows:

scala> val list = list (1,2,3,4) list:list[int] = List (1, 2, 3, 4) scala> list.map (x = x*2) Res0:list[int] = list ( 2, 4, 6, 8)

x = x*2 is an anonymous function. Each element in the list is applied to this function.

This is an anonymous function with only one parameter, and you can use _ placeholders to simplify the notation.

Scala> List.map (_*2) res1:list[int] = List (2, 4, 6, 8)

Scala> Val bigdata =list ("Spark", "Hadoop", "HBase") bigdata:list[string] = List (Spark, Hadoop, HBase) scala> Bigdata.map (_.length) Res2:list[int] = List (5, 6, 5)

Scala> Bigdata.map (_.tolist) Res3:list[list[char]] = List (list (s, p, a, R, K), List (H, a, D, O, O, p), list (H, B, a, S, e))

We'll print every word in the bigdata in a flashback.

Scala> Bigdata.map (_.tolist.reverse.mkstring) res6:list[string] = List (Kraps, Poodah, Esabh)

Flatmap function:

The "FlatMap" function has half the same function as the map function, but there is a requirement that the returned value of the incoming function be a list (should be seq) after processing, and if the result is not a list (seq), an error occurs. In other words, the function passed in is required-the return value is a SEQ line. Thus, after each element is processed, a list is returned, and we get a list,flatmap that contains a list element that automatically pulls all the elements of the internal list to form a list return.

Scala> List.flatmap (_*2) <console>:12:error:type mismatch;       Found:int required:scala.collection.gentraversableonce[?] List.flatmap (_*2)

Because _*2 return is not seq, so the error.

Scala> List.flatmap (_ match {case x = = List (x*2)}) Res5:list[int] = List (2, 4, 6, 8)

The foreach function:

The parameter is also a function that applies this parameter function to each element in the list.

scala> var sum =0sum:int = 0scala> list.foreach (sum +=_) scala> println (sum) 10

Filter function:

Returns a list whose arguments are a function that returns a Boolean type, or returns a value of True if the return.

If we want to keep the even numbers in the list, we can use the following methods:

Scala> List.filter {x = x%2 = = 0}res9:list[int] = List (2, 4)

This article is from the "Ding Dong" blog, please be sure to keep this source http://lqding.blog.51cto.com/9123978/1742034

35th: List of map, FlatMap, foreach, filter Operation code Combat