Re: alist and multiple values for one key

[Klaus Berndl <klaus.berndl@sdm.de>]

On 20 Jan 2003, Norbert C. wrote:



>  Hi,
>  
>         I would lile to find the "good way" to retrieve multiple
values
>  for a given key in an alist.
>  
>         But I didn't find a natural way to do it. For example :
>  (setq trees '((pine . cones) (pine . acorns) (oak . acorns) (maple .
>  seeds)))
>  ==> ((pine . cones) (pine . acorns) (oak . acorns) (maple . seeds))
>  (assoc 'pine trees)
>  ==> (pine . cones)
>  
>  What I'd like is something that returns each value associated with
>  'pine.
>  
>  Any thought ?

Multimaps are unfortunately not build in in Emacs. But you can implement
it
very easy. Here is a first fast hack how you can do it, probably not
robust
and fancy enough but it gives you a first impression:

,----
| ;; a fast hack for a multimap
| (defvar multimap nil)
| 
| (defun multimap-add (key value)
|   (let ((elem (assoc key multimap)))
|     (if elem
|         (when (not (member value (cdr elem)))
|           (setcdr elem (append (list value) (cdr elem))))
|       (setq multimap (cons (list key value) multimap)))
|     multimap))
| 
| (defun multimap-get (key)
|   (interactive "sKey: ")
|   (message "%s" (cdr (assoc key multimap))))
| 
| 
| ;; some tests
| (multimap-add "key1" "value1-1")
| (multimap-add "key1" "value1-2")
| (multimap-add "key2" "value2-1")
| (multimap-add "key2" "value2-2")
| (multimap-add "key2" "value2-3")
| (multimap-add "key3" "value3-3")
`----

Klaus

-- 
Klaus Berndl			mailto: klaus.berndl@sdm.de
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