Leandro Rios wrote:

> Leandro Rios escribió:
>
> > So this new version works as intended:
>
> > (defun cluster-by (fn list &aux (clusters (make-hash-table)) (result nil))
> > (mapcar #'(lambda (x) (push x (gethash (funcall fn x) clusters))) list)
> > (maphash #'(lambda (key value) (push value result)) clusters)
> > (sort result #'< :key #'(lambda (x) (length (car x)))))
>
> Um, sorry:
>
> (defun cluster-by (fn list &aux (clusters (make-hash-table)) (result nil))
> (mapcar #'(lambda (x) (push x (gethash (funcall fn x) clusters))) list)
> (maphash #'(lambda (key value) (push value result)) clusters)
> (sort result #'< :key #'(lambda (x) (funcall fn (car x)))))
> ^^^^^^^^^^

newLISP

(macro (aalt! Alist Key Func Default)
(if (assoc Key Alist)
(setf (assoc Key Alist) (list ($it 0) (Func ($it 1))))
(push (list Key (Func Default)) Alist)))

(macro (apush! Alist Key Val)
(aalt! Alist Key (curry cons Val) '()))

(define (group-by func lst (alist '()))
(dolist (x lst) (apush! alist (func x) x))
alist)

(group-by length '("a" "b" "abc" "bc" "a" "abcd" "e" "fg"))
===>
((4 ("abcd")) (2 ("fg" "bc")) (3 ("abc")) (1 ("e" "a" "b" "a")))