From: Stephane Chazelas <Stephane_Chazelas@yahoo.fr>
To: zsh-workers@sunsite.dk, "Jun T." <takimoto-j@kba.biglobe.ne.jp>
Subject: Re: arithmetic operator precedence
Date: Thu, 19 Jun 2008 17:20:36 +0100 [thread overview]
Message-ID: <20080619162036.GF5016@sc.homeunix.net> (raw)
In-Reply-To: <20080619160024.GK10734@prunille.vinc17.org>
On Thu, Jun 19, 2008 at 06:00:24PM +0200, Vincent Lefevre wrote:
> On 2008-06-19 10:54:54 +0100, Stephane Chazelas wrote:
> > With all the existing operators, one can do
> >
> > x=$(( some-expression ))
> > y=$(( some-other-expression ))
> > z=$(( $x <op> $y ))
> >
> > and it's OK whatever some-expression and some-other-expression
> > and <op> as long as they are POSIX.
>
> This is not guaranteed to work. Indeed POSIX allows constants other
> than the "canonical" ones to be recognized. With such constants,
> you can get wrong results. Whether such constants can occur for some
> reason is another matter. In *practice*, it is safer to write:
>
> z=$(( ($x) <op> ($y) ))
>
> and even safer to write:
>
> z=$(( x <op> y ))
>
> though it may not work in non-POSIX shells.
I'm under the impression your misunderstand the purpose of
POSIX. POSIX is a tool to help people write portable
applications (when it comes to shells, that means _scripts_).
You cannot write z=$(( x <op> y ))
in a POSIX script, because the result is unspecified.
z=$(($x <op> $y))
is specified (with the conditions I detailed).
If in your script, you make sure $x and $y are integer constants
(as in [-+]?(0[xX][a-fA-F0-9]+|0[0-7]+|[1-9][0-9]*)), then it
will work as expected.
If $x and $y are valid arithmetic expressions that may not be
integer constants as described above, then you need:
z=$((($x) <op> ($y)))
But I wouldn't do such things in a script.
My example was about $x and $y being the result of arithmetic
expansions.
[...]
> Shells should be designed to work *in practice* (and do what the
> user expects to get), not to work specifically on theoretical
> implementations that will never exist.
[...]
POSIX is not about a theoretical implementation. It's the
specification of an API or a language if you like.
Just like the C spec doesn't specify a theoretical compiler, but
a language. Of course, someone who wants to design a C compiler
must refer to that spec, but the main target of that spec is
people writing C code.
--
Stéphane
next prev parent reply other threads:[~2008-06-19 16:21 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 [this message]
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
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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