From: Stephane Chazelas <Stephane_Chazelas@yahoo.fr>
To: Zsh hackers list <zsh-workers@sunsite.dk>,
Richard Hartmann <richih.mailinglist@gmail.com>,
Peter Stephenson <p.w.stephenson@ntlworld.com>
Subject: Re: arithmetic operator precedence
Date: Tue, 17 Jun 2008 15:53:38 +0100 [thread overview]
Message-ID: <20080617145338.GQ5016@sc.homeunix.net> (raw)
In-Reply-To: <20080617143357.GO10734@prunille.vinc17.org>
On Tue, Jun 17, 2008 at 04:33:57PM +0200, Vincent Lefevre wrote:
[...]
> > which I understand as any occurrance of a variable name (other than
> > $-, $?, $0... obviously) in $((...)) should be the same as if the $
> > was not ommited (when $x contains an integer constant).
>
> No, POSIX doesn't say that. This sentence is a mean to define the
> value of x from the contents of $x. But note that parsing has already
> been done and you have something like a C expression ("The arithmetic
> expression shall be processed according to the rules given in
> Arithmetic Precision and Operations"); variables are just replaced
> by their values, like in C. Without any extension, both interpretations
> are equivalent anyway.
That's one interpretation.
[...]
> Also ** is out of the scope of POSIX too.
Yes, as I said I think clearly enough, if POSIX were to specify
** _in a future version of the standard_, it is more likely that
it implements it as -3 ** 2 == 9, to avoid confusion.
- because it doesn't at the moment clearly state how bare
variable names are to be interpreted as you point out (it's true
that I was interpreting maybe a bit too much)
- because it doesn't clearly state whether -3 is an integer
constant or not. ash allows a=-3; echo $((a * 3)) while it
doesn't allow a=0-3; echo $((a * 3)) for instance. But then,
given that a negative number can be the result of an
arithmetic expansion, I can't see how it can be interpreted in
any other way than -3 is an integer constant.
- because all existing shell implementations do it that way at
the moment.
> > No, POSIX does say that $((a ** 2)) is the same as $(($a ** 2))
> > because $a contains an integer constant, and that's $((-3**2)).
>
> No (see above). You are 3 times wrong. POSIX doesn't say anything
> about **, POSIX doesn't say that non-trivial expressions $((a ...))
> and $(($a ...)) are equivalent (and in pratice, they aren't), POSIX
> doesn't say anything about negative constants (probably a bug, but
> the whole section needs to be improved anyway). Perhaps instead of
> saying falsehood, you should read the standards.
[...]
No need to be rude.
Now, I think this has probably been discussed enough by now.
What is certain is that I will keep writing x=$(($x + 1)) and
make sure all the variables I use in arithmetic expansions
contain integer constants (negative or not), and if for some
obscure reason, I want to write a non-POSIX script meant to be
portable to ksh93, bash and zsh, I'll write
x=$((($y)**3))
as some shell maintainer might want to follow your advice in the
future and change the behavior so that
x=$(($y**3)) doesn't work anymore as I would expect it to.
I'll stop digressing on this on the list, please feel free to
follow up in private if you're keen.
--
Stéphane
next prev parent reply other threads:[~2008-06-17 14:53 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-06-12 9:57 Stephane Chazelas
2008-06-12 13:12 ` Mikael Magnusson
2008-06-12 13:40 ` Peter Stephenson
2008-06-12 14:47 ` Bart Schaefer
2008-06-12 15:01 ` Stephane Chazelas
2008-06-16 8:17 ` Vincent Lefevre
2008-06-16 8:07 ` Vincent Lefevre
2008-06-16 13:42 ` Peter Stephenson
2008-06-16 13:59 ` Stephane Chazelas
2008-06-16 14:33 ` Vincent Lefevre
2008-06-17 9:19 ` Richard Hartmann
2008-06-17 9:45 ` Stephane Chazelas
2008-06-17 10:24 ` Richard Hartmann
2008-06-17 10:24 ` Richard Hartmann
2008-06-17 10:38 ` Stephane Chazelas
2008-06-17 10:43 ` Peter Stephenson
2008-06-17 11:28 ` Vincent Lefevre
2008-06-17 11:46 ` Peter Stephenson
2008-06-17 12:05 ` Vincent Lefevre
2008-06-19 9:37 ` Jun T.
2008-06-19 9:54 ` Stephane Chazelas
2008-06-19 16:00 ` Vincent Lefevre
2008-06-19 16:20 ` Stephane Chazelas
2008-06-19 17:14 ` Vincent Lefevre
2008-06-19 9:58 ` Peter Stephenson
2008-06-19 12:29 ` Richard Hartmann
2008-06-19 16:04 ` Vincent Lefevre
2008-06-19 16:10 ` Mikael Magnusson
2008-06-19 16:27 ` Stephane Chazelas
2008-06-19 17:25 ` Vincent Lefevre
2008-06-19 17:20 ` Vincent Lefevre
2008-06-17 10:45 ` Richard Hartmann
2008-06-17 11:38 ` Vincent Lefevre
2008-06-17 11:19 ` Vincent Lefevre
2008-06-17 11:57 ` Stephane Chazelas
2008-06-17 12:35 ` Vincent Lefevre
2008-06-17 12:46 ` Stephane Chazelas
2008-06-17 13:02 ` Vincent Lefevre
2008-06-17 13:20 ` Stephane Chazelas
2008-06-17 14:33 ` Vincent Lefevre
2008-06-17 14:53 ` Stephane Chazelas [this message]
2008-06-17 15:49 ` Vincent Lefevre
2008-06-17 14:35 ` Stephane Chazelas
2008-06-17 15:05 ` Vincent Lefevre
2008-06-17 10:54 ` Vincent Lefevre
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