Re: Math expression evaluation error?

[Vincent Lefevre <vincent@vinc17.net>]

On 2015-01-14 02:03:33 +0300, ZyX wrote:
> 13.01.2015, 19:01, "Vincent Lefevre" <vincent@vinc17.net>:
> > In POSIX, it is always an integer division.
> 
> What?!
> 
> 1. How POSIX is related? Zsh is not a POSIX shell and it is not emulation mode that is being discussed here.

Zsh is partly based on POSIX for compatibility. The big difference
is the lack of word splitting (unless SH_WORD_SPLIT is set). Otherwise
I think that one should expect similar behavior, unless there is a
good reason.

> 2. If this standard is correct: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html#tagtcjh_15 then it references ISO C standard.
[...]
>    Where do you see a requirement for `/` to be integer division?

The context is an integer arithmetic. Thus / is necessarily an
integer division, like in C with integer types.

> 3. POSIX (http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04) says that “Only signed long integer arithmetic is required.” And the section referenced there talks about signed long or double. Signed long integer arithmetic is *required* to be in there, but signed long integer arithmetics is *not required* to be the *only* arithmetics present in POSIX shell.

It says: "as long as it does not affect the results in cases where
there is no overflow"

If $((1/2)) gave 0.5 in some shells, I fear this would break various
scripts.

So, if a real-floating type is used, then for "/", I suppose that
the implementation should do a floor() after the division if the
context has operands recognized as integers, which is here equivalent
to the integer division despite the two roundings (proved in
http://www.vinc17.net/research/papers/rr_intdiv).

But POSIX doesn't specify the arithmetic evaluation on expressions
other than signed long integer arithmetic. An implementation that
decides that $((1.0/2)) gives 17 "as an extension" could still be
conforming.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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