Re: alist and multiple values for one key

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

Hannu Koivisto <azure@iki.fi> writes:

> Friedrich Dominicus <frido@q-software-solutions.com> writes:
> 
> > 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))
> 
> I'm afraid that is not what your example expression evaluates to.
> Did you try it? 
Yes I tried it. I can not see why mapcan should be used here. I'm
consing up a list if the predicate function yields true. So mapcar is
the right choice.


What I originally wanted was:
(defun find-all-matching (pred list)
  (delete-if #'(lambda (item) (not (funcall pred item))) list))

but that won't work with Emacs Lisp but this will:
(defun find-all-matching (pred list)
  (delete-if `(lambda (item) (not (,pred item))) list))

So I suggest you tell me why mapcar is not the proper
function to use. 

Regards
Friedrich







> Learn about mapcan 


if you want to get your code
> right using one of the map... functions.  Of course, to return
> just the values as requested by the OP you will need other changes
> as well.
> 
> -- 
> Hannu