converting a list to map in scala

Question

I am trying to convert a list to map in scala.

Input

val colNames = List("salary_new", "age_new", "loc_new")

Output

Map(salary_new -> salary, age_new -> age, loc_new -> loc)

Following code is working, but seems like I am over killing it.

 val colRenameMap = colNames.flatMap(colname => Map(colname -> colname.split("_")(0))).toMap

Well, you can change the flatMap with a map and remove the creation of the inner Map. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented May 23, 2020 at 4:07
That worked. I was trying to do the same thing, but I had Map and it didn't worked. Not able to understand why? — Gaurang Shah
– Gaurang Shah, Commented May 23, 2020 at 4:11
map is defined like def map[A, B](fa: F[A])(f: A => B): F[B] (F here is List, Map, Set, Vector, etc) so it takes a function and applies it to every element inside the collection. Whereas def flatMap[A, B](fa: F[A])(f A => F[B]): F[B] so it is like map, but it is aware of the intermediate collections and flatten them. So, if you lave the inner Map you construct a list of maps, which is not what you want. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented May 23, 2020 at 4:21
I mean this one ` colNames.map(colname => Map(colname -> colname.split("_")(0)))` I was not able to apply toMap to this without using flatMap — Gaurang Shah
– Gaurang Shah, Commented May 23, 2020 at 4:29
Yes, it won't work. toMap transforms a list of tuples into a map, not a list of maps into a map. So, list.map(Map) returns a list of maps, List.flatMap(Map) returns a list of tuples and list.map(Tuple) also returns a list of tuples. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented May 23, 2020 at 4:38

abiratsis · Accepted Answer · 2020-05-23 16:33:17Z

1

I think map instead of flatMap would be more suitable for your case. Also you don't need to use the Map type internally, a single tuple should do the job.

For the sake of completeness this is how the definition of toMap looks like:

toMap[T, U](implicit ev: A <:< (T, U)): immutable.Map[T, U]

as you can see the method expects a (T, U) which is a Tuple2.

Finally, two options using map:

// option 1: key/value
colNames.map{c => c -> c.split("_")(0)}.toMap

// option 2: tuple
colNames.map{c => (c, c.split("_")(0))}.toMap

edited May 23, 2020 at 16:33

answered May 23, 2020 at 15:09

abiratsis

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