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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: Any way to collect all the values of a hash table more concisely ?
Date: Fri, 16 Aug 2024 09:17:30 -0000 (UTC)
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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")))