Re: alist and multiple values for one key

[Friedrich Dominicus <frido@q-software-solutions.com>]

ncauderan@hotmail.com (Norbert C.) writes:

> 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)
That does not work this way. assoc can only be used with a symbol but
if you step into the list you just have other lists. 

This code will work for the given data:
(defun collect-if (pred list)
  (mapcar #'(lambda (el) (when (funcall pred el) el)) list))

you can call it with
(collect-if #'(lambda (item) (eq (car item) 'pine)) trees)

((pine . cones) (pine . acorns))

Regards
Friedrich