How to concatenate list of strings with map reduce in scala

Question

I have a list of list of strings and I want to concatenate all the unique strings into a single (delimited space) string, something that flatMap allows one to do. However I am confused about the correct usage of the reduce function when concatenating 2 strings.

Input:

val listStrings: List[List[String]] = List(List("this", "is", "the"), List("this", "here"))

Expected Output (the order does not matter): String("this is the here")

I can only talk about the Java side of things (as this *is* tagged as Java). My first instinct would tell me to collect the the values into a set, then join by a space — EDToaster, Jun 24 '19 at 21:50

score 1 · Answer 1 · answered Jun 24 '19 at 22:09

If you have colossal amounts of repeating strings to deduplicate, you likely need two steps:

Map a list of strings to a set of strings (a trivial constructor invocation).
Reduce a sequence of sets into one set (maybe even in-place merging is possible).

If you only have 2-3 entries, as shown in your example, @jwh's answer in comments suffices.

How to concatenate list of strings with map reduce in scala

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