How to get rid of duplicates in a list, but keep the order

Question

I am using Intermediate Student with Lambda in DrRacket, I was wondering how one would remove the duplicates in a list, while keeping the order. For example (remo

Answer 1

SRFI-1 has delete-duplicates, although it's inefficient. (I am not too familiar with Racket, but surely it has SRFI-1, and the source...)

http://srfi.schemers.org/srfi-1/srfi-1.html#delete-duplicates

Answer 2

Run through the list sequentially, inserting each element in a hash table or other dictionary. If you try to insert an element that is already in the hash table, do not add it to the outgoing list.

Answer 3

If your goal is to get the functionality working, and not some homework question, then you don't need to do anything, just use remove-duplicates:

Welcome to Racket v5.2.
-> (remove-duplicates (list 2 5 4 5 1 2))
'(2 5 4 1)

Answer 4

This is the solution:

(define (remove-duplicates lon)
  (foldr (lambda (x y) (cons x (filter (lambda (z) (not (= x z))) y))) empty lon))

Answer 5

hmm i just had a racket exam recently, :/

the 'standard' remove-duplicates works fine but i was using pretty-big in drRacket so it had to be loaded using (require racket/list)

here is an alternative way :)

using mutation (not really in the spirit of racket but.. it works.)

    (define (set l)
        (define the-set '())
            (begin (for-each
                       (lambda (x)
                            (if (member x the-set)
                                #t
                            (set! the-set (cons x the-set))))
                       l)
                   (reverse the-set)))

hope this helps... cheers!

Answer 6

What you need to do is compare in reverse order the entire time. You can use the reverse function which returns a list in reverse order. That way you are always removing the 2nd+ occurrence of an element and not the first. Here is an example, however it is using let and if and not a cond expression.

http://www.cs.bgu.ac.il/~elhadad/scheme/duplicates.html

Good luck with your homework :)