Re: alist and multiple values for one key

[Hannu Koivisto <azure@iki.fi>]

kai.grossjohann@uni-duisburg.de (Kai Großjohann) writes:

> ncauderan@hotmail.com (Norbert C.) writes:
>
>>        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.
>
> Oh, with (require 'cl), you could do
>
> (remove-if-not (lambda (x) (eq (car x) KEY)) ALIST)
>
> Is that natural?  It's short, anyway.

If all your predicate does is (comparison-function (key-retriever x) key),
you can use the :key and :test{,-not} parameters of plain remove*,
resulting to a still shorter alternative:

(remove* key alist :key #'car :test-not #'eq)

But neither of these retrieve all values for a given key, which is
what the OP wanted.  Instead, they evaluate to alists with elements
whose keys match the given key.

So, something like this could be used to do what the OP wanted
(loop is from the cl package as well):

(loop for (key . val) in trees
      when (eq 'pine key) collect val)

-- 
Hannu