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From: "B. Pym" <No_spamming@noWhere_7073.org>
Newsgroups: comp.lang.lisp
Subject: Re: Any way to collect all the values of a hash table more concisely ?
Date: Fri, 5 Jul 2024 22:09:09 -0000 (UTC)
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> > 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))))) 
>                                           ^^^^^^^^^^

"lambda" is a macro that expands to (function (lambda ...)).
So instead of
  #'(lambda
he ought to use
  (lambda

The poster prefers #'lambda because he prefers ugliness.

Common Lisp is used by those who want code to be as
hideous as possible.

Gauche Scheme

(define (cluster-by fn lst)
  (let ((clusters (make-hash-table 'equal?)))
    (dolist (k lst)
      (hash-table-push! clusters (fn k) k))
    (hash-table-values clusters)))

(cluster-by string-length '("a" "b" "abc" "bc" "a" "abcd" "e" "fg"))
  ===>
(("abcd") ("e" "a" "b" "a") ("fg" "bc") ("abc"))