Re: Math expression evaluation error?

[Vincent Lefevre <vincent@vinc17.net>]

On 2015-01-15 00:37:43 +0300, ZyX wrote:
> 14.01.2015, 17:48, "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.
> 
> No. The context explicitly says that signed long integers or doubles
> should be used. Context of the ISO C standard obviously does not say
> anything like this.

Wrong. The context of $((...)) is signed long arithmetic. If a shell
wants to use double's (such as 1.0), that's only an extension.

> > 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.
> 
> Nope, it cannot ever be conforming. POSIX does this by saying that
> semantic of operation is the same as in C,

Only for what it specifies.

> except that only signed long integers and doubles are allowed.

No, POSIX says "As an extension, the shell may recognize arithmetic
expressions beyond those listed." and does not specify anything on
such expressions. $((1.0/2)) is such an arithmetic expression beyond
those listed.

In particular, it does *not* say that "only signed long integers and
doubles are allowed". A shell could implement other types (number
fields or whatever), and specific operators. And note that in such
an expression, "/" is not necessarily an operator, but could be part
of a constant.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)