How to remove duplicates from list without using in inbuilt libraries such as distinct, groupBy(identity), toSet.. Etc

Question

I wanted to write a Scala program that takes command-line args as list input and provide the output list without duplicates. I want to know the custom implementation of this without using any libraries.

Input : 4 3 7 2 8 4 2 7 3 Output :4 3 7 2 8

What standard library methods do you consider acceptable? `foldLeft`? What about types (`List`, `Set`)? Also, what have you tried so far? — Brian McCutchon, Dec 04 '18 at 06:56
Possible duplicate of [How in Scala to find unique items in List](https://stackoverflow.com/questions/1538598/how-in-scala-to-find-unique-items-in-list) — Brian McCutchon, Dec 04 '18 at 07:00
I must know, why not use something like `toSet` or `distinct`? — James Whiteley, Dec 04 '18 at 09:35
It was a assignment to write code manually without internal libraries.. I'm new to scala — , Dec 04 '18 at 14:07
Ah fair enough. You could do this quite nicely with recursion if you see my answer. — James Whiteley, Dec 07 '18 at 14:46

score 2 · Accepted Answer · answered Dec 04 '18 at 06:35

val x= List(4, 3, 7, 2, 8, 4, 2, 7, 3)
x.foldLeft(List[Int]())((l,v)=> if (l.contains(v)) l else  v :: l)

if you can't use contains you can do another fold

x.foldLeft(List[Int]())((l,v)=> if (l.foldLeft(false)((contains,c)=>if (c==v ) contains | true else contains | false)) l else  v :: l)

score 0 · Answer 2 · answered Dec 07 '18 at 14:59

Here's a way you could do this using recursion. I've tried to lay it out in a way that's easiest to explain:

import scala.annotation.tailrec

@tailrec
def getIndividuals(in: List[Int], out: List[Int] = List.empty): List[Int] = {
  if(in.isEmpty) out
  else if(!out.contains(in.head)) getIndividuals(in.tail, out :+ in.head)
  else getIndividuals(in.tail, out)
}

val list = List(1, 2, 3, 4, 5, 4, 3, 5, 6, 0, 7)
val list2 = List(1)
val list3 = List()
val list4 = List(3, 3, 3, 3)

getIndividuals(list) // List(1, 2, 3, 4, 5, 6, 0, 7)
getIndividuals(list2) // List(1)
getIndividuals(list3) // List()
getIndividuals(list4) // List(3)

This function takes two parameters, in and out, and iterates through every element in the in List until it's empty (by calling itself with the tail of in). Once in is empty, the function outputs the out List.

If the out List doesn't contain the value of in you are currently looking at, the function calls itself with the tail of in and with that value of in added on to the end of the out List.

If out does contain the value of in you are currently looking at, it just calls itself with the tail of in and the current out List.

Note: This is an alternative to the fold method that Arnon proposed. I personally would write a function like mine and then maybe refactor it into a fold function if necessary. I don't naturally think in a functional, fold-y way so laying it out like this helps me picture what's going on as I'm trying to work out the logic.

How to remove duplicates from list without using in inbuilt libraries such as distinct, groupBy(identity), toSet.. Etc

2 Answers2