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Revision: 6145
at April 30, 2008 14:23 by jarnaldich
Initial Code 
;; it requires srfi-1 (lists), for fold-right.
(define (alist-group-keys alist)
  (let ((create-cons-cell-or-append-to-existing 
         (lambda (current acum) 
           (let* ((key (car current))
                  (value (cdr current))
                  (cell (assoc key acum)))
             (if cell ; Key already seen, append current value to the list
                 (begin
                   (set-cdr! cell (cons value (cdr cell)))
                   acum)
                 ;else
                 (cons (list key value) acum))))))
    (fold-right create-cons-cell-or-append-to-existing '() alist)))

;; Example. Given an alist of authors and books:
;
; (alist-group-keys '(("Roth, Henry" . "Call It Sleep")
;                     ("Roth, Henry" . "Mercy of a Rude Stream")
;                     ("Houllebecq, Michel" . "Extension du domaine de la lutte")
;                     ("Houllebecq, Michel" . "Plateforme")))
;
;; It would return the books grouped by author:
;
;(("Roth, Henry" "Call It Sleep" "Mercy of a Rude Stream")
; ("Houllebecq, Michel" "Extension du domaine de la lutte" "Plateforme"))
;
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                                        Initial Description
                                        

                                    

                                    
Given an an associative list ((k1 . v1) (k2 . v2) ... (kn . vn)), with possibly equal values for ki, it returns another alist where all cells have distinct keys and where values originally associated to the same key have been grouped as a list in the new cons cell.  Ouch! This is much harder to explain than to code. Just check the commented example in the code.

                                

                                                            

                                    

                                        

                                        Initial Title
                                        

                                    

                                    
Grouping keys of an associative list

                                

                                                            

                                    

                                        

                                        Initial Tags
                                        

                                    

                                    
list

                                

                                                            

                                    

                                        

                                        Initial Language
                                        

                                    

                                    
Scheme