Re: Best way to handle mathematical divide-by-zero case

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: Best way to handle mathematical divide-by-zero case
Date	2017-02-07 15:58 +0100
Organization	A noiseless patient Spider
Message-ID	<o7cn88$d3a$1@dont-email.me> (permalink)
References	(10 earlier) <_I6mA.570101$Zf5.376281@fx41.am4> <o7b2ih$1mvi$1@gioia.aioe.org> <PK8mA.722159$Ww5.170172@fx42.am4> <o7c00v$qke$1@dont-email.me> <87fujqccyi.fsf@bsb.me.uk>

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On 07/02/17 15:06, Ben Bacarisse wrote:
> David Brown <david.brown@hesbynett.no> writes:
> 
>> On 07/02/17 01:43, JiiPee wrote:
> <snip>
>>> what if the average is calculated 10 million times per second? would you
>>> still ldo the check eatch time?
> <snip>
>> It is really quite simple, and I cannot understand why you are taking so
>> long to get the hang of it.
>>
>> Let me give you the /one/ rule that actually matters:
>>
>> * Do not divide by zero - it is a mistake in the code. *
>>
>> OK?
> 
> I don't think that's a good universal rule.  It very often a good rule,
> and if you are writing portable C it is an essential rule, but there are
> times when you can rely on behaviour outside of the C++ standard.  In
> those cases writing simpler code and letting IEEE floating point take
> care of the NaNs and the Infinities might be the way to go.
> 
> <snip>
> 

There is always going to be scope for making things more complicated to
suit particular cases - C and C++ are flexible languages, and compilers
often have a range of options or features that go beyond the bare
requirements of the standards.

However, I think unless you are an expert who is aware of all the
subtleties and has a solid understanding of the target platform(s),
compiler(s) and compiler options, then it /is/ a good rule.  And a
poster who wonders if it is okay to skip a check purely on the basis of
the number of times a function is to be called, is not such an expert
(in this particular area).

Actually, I'd go beyond that and keep my rule even for experts -
dividing by zero is a mistake (unless you are dealing with projective
planes and other fun maths).  Relying on features such as IEEE floating
point NaNs, infinities, or exceptions is one way of dealing with such
mistakes.  Sometimes it is more efficient to do your calculations hoping
there is no problem such as division by zero, with the intention of
detecting the problem afterwards, rather than checking for problems
/before/ doing the calculation.  That's fine - but you are not actually
dividing by zero any more.  You are doing speculative calculations that
may be aborted by exceptions, or calculations using functions specified
as returning NaN for a denominator of 0 or the quotient otherwise.  In
other words, you are using specified, defined behaviour - not the
undefined behaviour of a division by zero.

Thread

Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-11 18:17 +0000
  Re: Best way to handle mathematical divide-by-zero case "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2017-01-11 20:00 +0100
    Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-11 19:16 +0000
      Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 10:36 +0100
        Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 13:16 +0000
          Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 15:23 +0100
            Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:13 +0000
              Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 17:35 +0100
            Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:27 +0000
              Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 17:37 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:44 +0000
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:46 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-13 12:06 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-13 11:36 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-13 13:12 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-13 13:11 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-13 15:23 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-13 14:57 +0000
                Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-13 17:35 -0800
                Re: Best way to handle mathematical divide-by-zero case Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-13 20:25 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-14 18:08 +0100
                Re: Best way to handle mathematical divide-by-zero case Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-14 20:30 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-15 00:16 +0100
                Re: Best way to handle mathematical divide-by-zero case Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-15 00:34 +0000
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-15 01:01 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-15 16:57 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-15 17:17 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-15 19:13 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:48 +0000
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:51 +0000
              Re: Best way to handle mathematical divide-by-zero case Manfred <mx2927@gmail.com> - 2017-02-06 01:08 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-06 00:39 +0000
                Re: Best way to handle mathematical divide-by-zero case Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2017-02-06 00:50 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-02-06 02:01 +0100
                Re: Best way to handle mathematical divide-by-zero case Manfred <noname@invalid.add> - 2017-02-06 13:56 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-06 22:25 +0000
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-06 22:26 +0000
                Re: Best way to handle mathematical divide-by-zero case Robert Wessel <robertwessel2@yahoo.com> - 2017-02-06 16:46 -0600
                Re: Best way to handle mathematical divide-by-zero case Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2017-02-06 23:49 +0000
                Re: Best way to handle mathematical divide-by-zero case scott@slp53.sl.home (Scott Lurndal) - 2017-02-07 13:24 +0000
                Re: Best way to handle mathematical divide-by-zero case Manfred <mx2927@gmail.com> - 2017-02-07 00:58 +0100
                Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-07 00:43 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-02-07 09:22 +0100
                Re: Best way to handle mathematical divide-by-zero case Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-07 14:06 +0000
                Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-02-07 15:58 +0100
                Re: Best way to handle mathematical divide-by-zero case Öö Tiib <ootiib@hot.ee> - 2017-02-07 01:00 -0800
                Re: Best way to handle mathematical divide-by-zero case Manfred <noname@invalid.add> - 2017-02-07 15:31 +0100
                Re: Best way to handle mathematical divide-by-zero case scott@slp53.sl.home (Scott Lurndal) - 2017-02-07 13:22 +0000
        Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 13:20 +0000
          Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 15:28 +0100
            Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 15:40 +0000
              Re: Best way to handle mathematical divide-by-zero case David Brown <david.brown@hesbynett.no> - 2017-01-12 16:52 +0100
    Re: Best way to handle mathematical divide-by-zero case Wouter van Ooijen <wouter@voti.nl> - 2017-01-11 23:20 +0100
  Re: Best way to handle mathematical divide-by-zero case Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2017-01-11 21:43 +0000
    Re: Best way to handle mathematical divide-by-zero case Gareth Owen <gwowen@gmail.com> - 2017-01-12 19:12 +0000
  Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 00:07 +0000
  Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 00:12 +0000
  Re: Best way to handle mathematical divide-by-zero case Real Troll <real.troll@trolls.com> - 2017-01-12 12:55 -0400
    Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-12 16:53 +0000
  Re: Best way to handle mathematical divide-by-zero case Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2017-01-12 19:10 +0000
    Re: Best way to handle mathematical divide-by-zero case Gareth Owen <gwowen@gmail.com> - 2017-01-12 19:17 +0000
  Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-13 15:34 -0800
    Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-14 00:14 +0000
      Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-13 16:56 -0800
        Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-13 17:46 -0800
        Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-14 01:51 +0000
          Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-13 18:09 -0800
    Re: Best way to handle mathematical divide-by-zero case Gareth Owen <gwowen@gmail.com> - 2017-01-14 11:21 +0000
      Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-14 12:45 -0800
        Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-01-14 21:02 +0000
          Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-14 13:47 -0800
        Re: Best way to handle mathematical divide-by-zero case "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-01-14 13:03 -0800
  Re: Best way to handle mathematical divide-by-zero case "J. Clarke" <j.clarke.873638@gmail.com> - 2017-02-04 07:56 -0500
  Re: Best way to handle mathematical divide-by-zero case Jorgen Grahn <grahn+nntp@snipabacken.se> - 2017-02-05 10:21 +0000
    Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-05 13:33 +0000
      Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-06 07:33 +0000
    Re: Best way to handle mathematical divide-by-zero case JiiPee <no@notvalid.com> - 2017-02-05 13:34 +0000
      Re: Best way to handle mathematical divide-by-zero case Jorgen Grahn <grahn+nntp@snipabacken.se> - 2017-02-05 18:51 +0000

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