Stripping duplicate elements in a list of strings in elisp

Question

Given a list such as

(list "foo" "bar" nil "moo" "bar" "moo" nil "affe")

how would I build a new list with the duplicate strings removed, as well as the nils stripped, i.e.

(list "foo" "bar" "moo" "affe")

The order of the elements needs to be preserved - the first occurence of a string may not be removed.

The lists I'm dealing with here are short, so there's no need to use anything like a hash table for the uniqueness check, although doing so certainly wouldn't hurt either. However, using cl functionality is not a viable option.

Answer 1

Here ya go:

(defun strip-duplicates (list)
  (let ((new-list nil))
    (while list
      (when (and (car list) (not (member (car list) new-list)))
        (setq new-list (cons (car list) new-list)))
      (setq list (cdr list)))
    (nreverse new-list)))

Answer 2

The Common Lisp package contains many list manipulation functions, in particular remove-duplicates.

(require 'cl)
(remove-duplicates (list "foo" "bar" nil "moo" "bar" "moo" nil "affe")
                   :test (lambda (x y) (or (null y) (equal x y)))
                   :from-end t)

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Yes, I realize you said you didn't want to use cl. But I'm still mentioning this as the right way to do it for other people who might read this thread.

(Why is cl not viable for you anyway? It's been shipped with Emacs for about 20 years now, not counting less featured past incarnations.)

Answer 3

Try the "Lists" section of the Emacs Lisp Reference Manual:

(delq nil (delete-dups (list "foo" "bar" nil "moo" "bar" "moo" nil "affe")))