From: Stephen Berman <stephen.berman@gmx.net>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: help-gnu-emacs@gnu.org
Subject: Re: Why does using aset sometimes output raw bytes?
Date: Sun, 09 Dec 2018 21:43:50 +0100 [thread overview]
Message-ID: <87lg4ymnw9.fsf@gmx.net> (raw)
In-Reply-To: <jwv1s6q4isk.fsf-monnier+gmane.emacs.help@gnu.org> (Stefan Monnier's message of "Sun, 09 Dec 2018 14:20:07 -0500")
On Sun, 09 Dec 2018 14:20:07 -0500 Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>> I don't have a use case where using aset like this is indispensable, I
>> was just experimenting. Are your reservations because the
>> implementation of aset is brittle, leading to things like the
>> observations I reported -- maybe too hard to fix and not worth the
>> trouble?
>
> It's not the implementation, but the semantics of unibyte/multibyte
> strings presumes that the difference doesn't matter much for ASCII-only
> strings, which is mostly true but isn't true in the case of `aset`.
Yes, thanks; I also appreciate this better now after Eli's explanations.
> Also you probably expect `aset` to be constant-time, but on multibyte
> strings it can take time O(N) where N is the length of the string:
> Emacs's multibyte strings are designed for sequential access rather than
> random access, and since chars can take a variable amount of space,
> replacing one with another can require shifting things around and
> allocating a new chunk of memory.
Interesting. I was in fact wondering about just such issues because of
code posted here that permutes strings using split-string and sort,
which prompted me to try some alternatives, one of which was to use a
while-loop instead of sort and another was using a loop and aset instead
of split-string. I guess this is well-explored and I could probably do
a web search for the most efficient algorithm, but I really just wanted
to see what I could come up with in Emacs Lisp and so bumped into these
multibyte issues. So it's already been a useful learning experience.
>> Or are there other reasons not to use aset as above?
>
> In most cases `aset` results in more complex and more brittle code when
> working on strings. It's not always the case and the code without
> `aset` occasionally is a lot worse, admittedly, but as a first rule,
> I strongly recommend to stay away with it.
>
> You'll also gain karma points along the way,
Thanks for the feedback.
Steve Berman
prev parent reply other threads:[~2018-12-09 20:43 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-09 15:16 Why does using aset sometimes output raw bytes? Stephen Berman
2018-12-09 15:20 ` Eli Zaretskii
2018-12-09 15:46 ` Stephen Berman
2018-12-09 15:56 ` Stephen Berman
2018-12-09 17:12 ` Eli Zaretskii
2018-12-09 17:32 ` Stephen Berman
2018-12-09 17:47 ` Eli Zaretskii
2018-12-09 18:50 ` Stephen Berman
2018-12-09 18:55 ` Eli Zaretskii
2018-12-09 17:10 ` Stefan Monnier
2018-12-09 17:20 ` Stephen Berman
2018-12-09 19:20 ` Stefan Monnier
2018-12-09 20:23 ` Eli Zaretskii
2018-12-09 21:20 ` Stefan Monnier
2018-12-10 5:59 ` Eli Zaretskii
2018-12-10 13:56 ` Stefan Monnier
2018-12-09 20:43 ` Stephen Berman [this message]
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