Is there already an alist function which gets all matching elements, not just the first?

Is there already an alist function which gets all matching elements, not just the first?

[Alan Mackenzie]

Hello, Emacs.

I want to be able to get all matching elements from an alist (thinking
about extending imenu).  Something like

    (assoc-all key list)

which would return a list of matches.  After all, we have functions
which _delete_ all matches from an alist.

Does such a function already exist, perhaps in cl-*.el?

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Is there already an alist function which gets all matching elements, not just the first?

[Robert Pluim]

>>>>> On Wed, 5 Oct 2022 10:00:10 +0000, Alan Mackenzie <acm@muc.de> said:

    Alan> Hello, Emacs.
    Alan> I want to be able to get all matching elements from an alist (thinking
    Alan> about extending imenu).  Something like

    Alan>     (assoc-all key list)

    Alan> which would return a list of matches.  After all, we have functions
    Alan> which _delete_ all matches from an alist.

    Alan> Does such a function already exist, perhaps in cl-*.el?

Just use assoc-delete-all and invert the test:

(assoc-delete-all mykey myalist (lambda (val key)
                                  (not (eq val key))))

Robert
-- 

Re: Is there already an alist function which gets all matching elements, not just the first?

[Philip Kaludercic]

Alan Mackenzie <acm@muc.de> writes:

> Hello, Emacs.
>
> I want to be able to get all matching elements from an alist (thinking
> about extending imenu).  Something like
>
>     (assoc-all key list)
>
> which would return a list of matches.  After all, we have functions
> which _delete_ all matches from an alist.
>
> Does such a function already exist, perhaps in cl-*.el?

This appears to do the right thing

(map-filter (lambda (k _v) (eq k '3))
	    '((3 . 4)
	      (2 . 4)
	      (1 . 2)
	      (3 . 1)))
;; => ((3 . 4) (3 . 1))

Perhaps a `map-member' could be implemented that would do something like
what you are looking for?  Or does this already exist by some other name
(I don't really use map.el). I've certainly wanted something like this
more than a few times.

> Thanks!

Re: Is there already an alist function which gets all matching elements, not just the first?

[Lars Ingebrigtsen]

Philip Kaludercic <philipk@posteo.net> writes:

> Perhaps a `map-member' could be implemented that would do something like
> what you are looking for?  Or does this already exist by some other name
> (I don't really use map.el). I've certainly wanted something like this
> more than a few times.

There's `map-filter'...  Hm...  but it takes a predicate, which makes it
not as convenient as (say) `assq'.

So, yes, perhaps there should be a... `map-members'?  `map-all-members'?

Re: Is there already an alist function which gets all matching elements, not just the first?

[Alan Mackenzie]

Hello, Robert.

On Wed, Oct 05, 2022 at 12:45:00 +0200, Robert Pluim wrote:
> >>>>> On Wed, 5 Oct 2022 10:00:10 +0000, Alan Mackenzie <acm@muc.de> said:

>     Alan> Hello, Emacs.
>     Alan> I want to be able to get all matching elements from an alist (thinking
>     Alan> about extending imenu).  Something like

>     Alan>     (assoc-all key list)

>     Alan> which would return a list of matches.  After all, we have functions
>     Alan> which _delete_ all matches from an alist.

>     Alan> Does such a function already exist, perhaps in cl-*.el?

> Just use assoc-delete-all and invert the test:

> (assoc-delete-all mykey myalist (lambda (val key)
>                                   (not (eq val key))))

Thanks for the idea!  But I'd have to copy the alist first, and then it
starts to look a bit contrived.

Maybe I should just write a straightforward loop to get this info.

> Robert
> -- 

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Is there already an alist function which gets all matching elements, not just the first?

[Alan Mackenzie]

Hello, Philip.

On Wed, Oct 05, 2022 at 18:16:43 +0000, Philip Kaludercic wrote:
> Alan Mackenzie <acm@muc.de> writes:

> > Hello, Emacs.

> > I want to be able to get all matching elements from an alist (thinking
> > about extending imenu).  Something like

> >     (assoc-all key list)

> > which would return a list of matches.  After all, we have functions
> > which _delete_ all matches from an alist.

> > Does such a function already exist, perhaps in cl-*.el?

> This appears to do the right thing

> (map-filter (lambda (k _v) (eq k '3))
> 	    '((3 . 4)
> 	      (2 . 4)
> 	      (1 . 2)
> 	      (3 . 1)))
> ;; => ((3 . 4) (3 . 1))

The doc string for map-filter doesn't actually describe the function,
sadly.  It talks about key/value pairs whilst giving no clue where the
key and the value come from.  I suppose one might guess, but really
there's no alternative to studying the source code for map-filter.

> Perhaps a `map-member' could be implemented that would do something like
> what you are looking for?  Or does this already exist by some other name
> (I don't really use map.el). I've certainly wanted something like this
> more than a few times.

It seems like one of these things which quite a lot of people want, but
nobody wants it enough to be bothered to implement it.  ;-)

> > Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).