Re: How to get plist properties list?

[Jean Louis]

* Stefan Monnier <monnier@iro.umontreal.ca> [2021-01-08 08:23]:
> >     (dotimes (i length (reverse properties))
> >       (if (divisible-by-2-or-0-p i)
> >           (push (elt plist i) properties)))))
> 
> This is another example of a dotimes+elt loop, i.e. a loop where you
> end up with an unwarranted O(n²) complexity (i.e. a performance bug).

Yes, I know there are complexities, but it did not bother me really.
Only compiler warnings bother me.

> >     (while (/= n (length plist))
> >       (push (elt plist n) properties)
> >       (setq n (+ 2 n)))
> >     properties))
> 
> Same problem here.

I would like to understand what is the problem. I don't. You tell me
that `elt' is problem, that is how I understand it.

Could I maybe rather use `nth' to replace `elt'?

(elt '(1 2 3) 0)
(nth 0 '(1 2 3))

Which one is faster?

> > But maybe there is some more simpler way to get plist properties list?
> 
> Probably not the simplest but this should work:
> 
>     (defun plist-keys (plist)
>       (let (keys iskey)
>         (dolist (x plist)
>           (if (setq iskey (not iskey)) (push x keys)))
>         (nreverse keys)))

As we already discussed it, `dolist' is not perfect, it would give
warnings, not logical at all. In the above example `x' would not give
warning, but if there would be return value then `x'  would be part of
warning, while return variable would not be. That is not logical to
me, so I consider `dolist' not well handled by compiler and is better
for me not to use it.

> The simplest might be:
> 
>     (map-keys plist)

That is it. Why not start with that one...


Jean