Re: Collecting like-labelled sublists of a list

Madhu wrote:

|> (defun test (list)
|>   (loop for j in list
|>         for index = (first j)
|>         for k = (rest j)
|>         with indices = nil
|>         if (not (member index indices))
|>           do (pushnew index indices)
|>           and collect j into res
|>         else
|>           do (nconc (assoc index res) k) ; ASSOC instead of NTH
|>         finally (return res)))
|
| To be more precise (if that helps), I'm wondering if there's a way of
| doing this without having to build up a list of the indices (labels)
| and using membership/non-membership of this list as the test for
| whether we have encountered a new index or not.
 
You can get by without building indices and just using ASSOC (which you
cannot avoid):
 
(defun cortez-group (list)      ; Destroys LIST!
  (let (result)
    (dolist (el list)
      (let ((entry (assoc (car el) result)))
        (if entry
            (rplacd entry (nconc (cdr entry) (cdr el)))
            (push el result))))
    (nreverse (mapcar #'cdr result))))
 
* (setq $a '((0 a b) (1 c d) (2 e f) (3 g h) (1 i j)
             (2 k l) (4 m n) (2 o p) (4 q r) (5 s t)))
* (cortez-group $a)
=> ((A B) (C D I J) (E F K L O P) (G H) (M N Q R) (S T))

Gauche Scheme

(use srfi-235) ;; group-by

(define (c-group lst)
  (map
    (cut append-map cdr <>)
    ((group-by car) lst)))

(c-group '((0 a b) (1 c d) (2 e f) (3 g h) (1 i j)
             (2 k l) (4 m n) (2 o p) (4 q r) (5 s t)))

  ===>
((a b) (c d i j) (e f k l o p) (g h) (m n q r) (s t))