How to get unique elements from two lists of strings in scala? - scala

Question

I have two list to compare:
List one:
List("one","two","three","four")
List two:
List("one","two")
how can I get the unique values from these two lists?

Answer 1

If your two lists are r1 and r2, and assuming you want the values in each list that are not present in the other:
r1.filterNot(r2.contains) ::: r2.filterNot(r1.contains)
or
r1.diff(r2) ::: r2.diff(r1)

Answer 2

Turn them into sets, and get the intersection. You may then turn it back to Seq if you want, but first ask yourself if they had to be Seq in first place, instead of Set.
scala> List("one","two","three","four").toSet & List("one","two").toSet
res0: scala.collection.immutable.Set[String] = Set(one, two)

Answer 3

Use The difference operator for Set &~
http://www.scala-lang.org/api/current/scala/collection/immutable/Set.html

Answer 4

I use List(1, 2, 3, 4) ::: List(1, 2, 5) distinct for this issue. It returns List(1, 2, 3, 4, 5).

Answer 5

I would suggest using the following for O(m+n) running time (assumes input arrays are sorted).
def mergeUniqueSortedArrays( A: Array[String], B: Array[String] ): Array[String]= {
val n = A.length
val m = B.length
var C = Array[String]()
var i = 0
var j = 0
while (i < n && j < m) {
if (i == n) {
if ( B(j) != A(i-1) ) {
C :+= B(j)
}
j+=1
}
else if (j == m) {
if ( A(i) != B(j-1) ) {
C :+= A(j)
}
i+=1
}
else {
if ( A(i) < B(j) ) {
if (C.length == 0 || A(i) != C(C.length-1)) {
C :+= A(i)
}
i+=1
}
else if ( B(j) < A(i) ) {
if (C.length == 0 || B(j) != C(C.length-1)) {
C :+= B(j)
}
j+=1
}
else {
if (C.length == 0 || A(i) != C(C.length-1)) {
C :+= A(i)
}
i+=1
j+=1
}
}
}
return C
}
--
NOTE: If the input arrays are not sorted, then you can easily sort the input arrays and it will run in in O( max{(n + m), (n log n)}) time, assuming n>m.
NOTE: O(n + m) time technically assumes that string length is bounded by constant k, but you aren't going to get around that anyway.