[Numpy-discussion] Deprecate boolean math operators?

[Charles R Harris]

On Fri, Dec 6, 2013 at 2:14 PM, <josef.pktd at gmail.com> wrote:

> On Fri, Dec 6, 2013 at 3:50 PM, Sebastian Berg
> <sebastian at sipsolutions.net> wrote:
> > On Fri, 2013-12-06 at 15:30 -0500, josef.pktd at gmail.com wrote:
> >> On Fri, Dec 6, 2013 at 2:59 PM, Nathaniel Smith <njs at pobox.com> wrote:
> >> > On Fri, Dec 6, 2013 at 11:55 AM, Alexander Belopolsky <
> ndarray at mac.com> wrote:
> >> >>
> >> >>
> >> >>
> >> >> On Fri, Dec 6, 2013 at 1:46 PM, Alan G Isaac <alan.isaac at gmail.com>
> wrote:
> >> >>>
> >> >>> On 12/6/2013 1:35 PM, josef.pktd at gmail.com wrote:
> >> >>> > unary versus binary minus
> >> >>>
> >> >>> Oh right; I consider binary `-` broken for
> >> >>> Boolean arrays. (Sorry Alexander; I did not
> >> >>> see your entire issue.)
> >> >>>
> >> >>>
> >> >>> > I'd rather write ~ than unary - if that's what it is.
> >> >>>
> >> >>> I agree.  So I have no objection to elimination
> >> >>> of the `-`.
> >> >>
> >> >>
> >> >> It looks like we are close to reaching a consensus on the following
> points:
> >> >>
> >> >> 1. * is well-defined on boolean arrays and may be used in preference
> of & in
> >> >> code that is designed to handle 1s and 0s of any dtype in addition to
> >> >> booleans.
> >> >>
> >> >> 2. + is defined consistently with * and the only issue is the
> absence of
> >> >> additive inverse.  This is not a problem as long as presence of -
> does not
> >> >> suggest otherwise.
> >> >>
> >> >> 3. binary and unary minus should be deprecated because its use in
> >> >> expressions where variables can be either boolean or numeric would
> lead to
> >> >> subtle bugs.  For example -x*y would produce different results from
> -(x*y)
> >> >> depending on whether x is boolean or not.  In all situations, ^ is
> >> >> preferable to binary - and ~ is preferable to unary -.
> >> >>
> >> >> 4. changing boolean arithmetics to auto-promotion to int is
> precluded by a
> >> >> significant use-case of boolean matrices.
> >> >
> >> > +1
> >>
> >> +0.5
> >> (I would still prefer a different binary minus, but it would be
> >> inconsistent with a logical unary minus that negates.)
> >>
> >
> > The question is if the current xor behaviour can make sense? It doesn't
> > seem to make much sense mathematically? Which only leaves that `abs(x -
> > y)` is actually what a (python) programmer might expect.
> > I think I would like to deprecate at least the unary one. The ~ kind of
> > behaviour just doesn't fit as far as I can see.
>
> I haven't seen any real use cases for xor yet.
>

Using it instead of '+' yields a boolean ring instead of semi-ring. Papers
from the first quarter of the last century used it pretty often on that
account, hence 'sigma-rings', etc. Eventually the simplicity of the
inclusive or overcame that tendency.

My impression is that both plus and minus are just overflow accidents
> and not intentional. plus works in a useful way, minus as xor might be
> used once per century.
>

It's certainly weird given that '+' means the inclusive or. I think '^' is
much preferable.
Although it makes some sense if one can keep the semantics straight.
Complicated, though.


> I would deprecate both unary and binary minus.
>
> (And when nobody is looking in two versions from now, I would add a
> binary minus that overflows to the clipped version, so I get a set
> subtraction. :)
>

Where is '\' when you need it?

<snip>

Chuck
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