Re: arithmetic operator precedence

[Stephane Chazelas <Stephane_Chazelas@xxxxxxxx>]

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