Re: alist and multiple values for one key

[Friedrich Dominicus]

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 and unfortunatly did I get those results, which I know now are not
correct after your posting and re-thinking and re-trying it. 

However what I wanted to write was

(defun find-all-matching (pred list)
  (delete-if (lambda (item) (not (funcall pred item))) list))

But this won't work in Emacs Lisp but this (according to posting some days
ago:
(defun find-all-matching (pred list)
  (delete-if `(lambda (item) (not (,pred item))) list))


and indeed with list I got

(setf ttree '((pine . p1) (pine . p2) (oak . p3) (oak . p4) (pine
. p4)))

(find-all-matching #'(lambda (item) (eq (car item) 'pine)) ttree)
((pine . p1) (pine . p2) (pine . p4))

This is what I wanted to get. But it's nevertheless wrong because it
is not what the original author wanted. You are right. He
just wanted the values and there your example with loop works as it
should.

Thanks for pointing that out to me.

Friedrich