List of lists as Map value does not compile

Question

I have in Scala a Map where the values are list of lists. I try to add a value with the following code:

var map = Map[String,List[List[String]]]()
val list1 = List ("A111", "B111")
var listInMap = map.getOrElse("abc", List[List[String]]())
listInMap += list1   // this line does not compile
map += ("abc" -> listInMap)

The problem is that in the line listInMap += list1 it throws type mismatch; found : List[String] required: String. Why is String required if I need to add a list to the list? I need to add list1 to listInMap

score 5 · Accepted Answer · edited May 23 '17 at 12:33

5

listInMap += list1 is equivalent to listInMap = listInMap + list1. + operator is not defined in List for latest scala library (2.11.8) (marked deprecated in 2.7). So, + operator just concat the string values of listInMap and list1 with latest scala library.

For latest scala, you can use, listInMap = listInMap :+ list1

Also, check this: https://stackoverflow.com/a/7794297/1433665 as appending to list in scala has complexity of O(n)

edited May 23 '17 at 12:33

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answered Aug 23 '16 at 02:05

TheKojuEffect

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Doesn't `listInMap += list1` add `list1` to `listInMap` and not vice versa? I need to add `list1` to `listInMap` – ps0604 Aug 23 '16 at 02:11
Hey @ps0604, Yes, you are right. check my updated answer. I hope it is helpful. – TheKojuEffect Aug 23 '16 at 02:27
Now I'm getting this error: `type mismatch; found : List[java.io.Serializable] required: List[List[String]]` – ps0604 Aug 23 '16 at 03:44

score 1 · Answer 2 · answered Aug 23 '16 at 04:04

1

Your problem is that list1 is not being inserted at the correct depth/level.

This will compile.

. . .
map += ("abc" -> (listInMap ++ List(list1)))
map("abc")  // result: List[List[String]] = List(List(A111, B111))

answered Aug 23 '16 at 04:04

jwvh

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List of lists as Map value does not compile

2 Answers2