Re: Detect Math Errors in Functions

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: Detect Math Errors in Functions
Date	2021-09-01 19:25 +0200
Organization	A noiseless patient Spider
Message-ID	<sgod26$pam$1@dont-email.me> (permalink)
References	<hOudnfXpudHWjbL8nZ2dnUU7-KHNnZ2d@giganews.com> <s88uig9ljth6ueakm5lsgsh4rksmat3h5u@4ax.com> <sgnbcu$3v7$1@dont-email.me> <9o6dnYFV2J_wA7L8nZ2dnUU78XHNnZ2d@brightview.co.uk>

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On 01/09/2021 17:38, Mike Terry wrote:
> On 01/09/2021 08:50, David Brown wrote:
>> On 01/09/2021 08:54, Barry Schwarz wrote:
>>> On Wed, 1 Sep 2021 07:31:53 +0200, Joseph Hesse <joeh@gmail.com>
>>> wrote:
>>>
>>>> The famous unproven Collatz math conjecture says that if you start with
>>>> any positive integer n and generate a sequence by repeatedly apply the
>>>> rule that if n is even the next term is n/2, otherwise the next term is
>>>> 3*n + 1, then eventually the sequence will reach 1.
>>>>
>>>> I wrote a C++ function that will generate the sequence from any
>>>> positive
>>>> starting integer, see below, header files omitted.  Since there is
>>>> integer arithmetic involved, there might be a math overflow.  How does
>>>> one write code to detect math underflow or overflow errors in functions
>>>> that do math computations?
>>>>
>>> In this particular case, it seems to me to be easier to prevent the
>>> overflow before it occurs rather than try to detect it after it
>>> occurs.  You could replace the previous assignment with
>>
>> I agree with that.  I've always preferred the "look before you leap"
>> approach, rather than the "close your eyes, make the jump and let
>> someone else sweep up the mess" tactic.
>>
>> (The rest is to the OP, rather than Barry.)
>>
>> Another option is to use gcc's overflow-detecting builtins, or other
>> equivalent extensions for other compilers.  If you are trying to do this
>> checking as fast as possible, you might be looking at SIMD support
>> through compiler vector extensions or processor-specific intrinsics.
>>
>> For unsigned types, you can of course just do "y = 3 * x + 1;" and check
>> for "y < x", as the overflow is defined as wrapping.
> 
> That kind of test works for pure addition, but not when multiplication
> is involved.
> 
> For example, mod 16 we have if x = 8, then y = 3*8 + 1 = 9, and y >= x.
> So your test would fail, but there was overflow!  Barry's test is the
> way I've gone in the past, or use some library class built for the
> purpose of overflow detection.
> 
> 

You are right, of course - sorry for posting without thinking here.

Thread

Detect Math Errors in Functions Joseph Hesse <joeh@gmail.com> - 2021-09-01 07:31 +0200
  Re: Detect Math Errors in Functions Barry Schwarz <schwarzb@delq.com> - 2021-08-31 23:54 -0700
    Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-01 09:30 +0200
      Re: Detect Math Errors in Functions Barry Schwarz <schwarzb@delq.com> - 2021-09-01 07:53 -0700
        Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-01 17:50 +0200
        Re: Detect Math Errors in Functions James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-01 12:28 -0400
        Re: Detect Math Errors in Functions Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-09-01 17:38 +0100
    Re: Detect Math Errors in Functions David Brown <david.brown@hesbynett.no> - 2021-09-01 09:50 +0200
      Re: Detect Math Errors in Functions Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-09-01 16:38 +0100
        Re: Detect Math Errors in Functions David Brown <david.brown@hesbynett.no> - 2021-09-01 19:25 +0200
          Re: Detect Math Errors in Functions Hope Rouselle <hrouselle@jevedi.com> - 2021-09-06 11:04 -0300
      Re: Detect Math Errors in Functions Cholo Lennon <chololennon@hotmail.com> - 2021-09-06 00:20 -0300
        Re: Detect Math Errors in Functions "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-09-06 07:23 +0200
          Re: Detect Math Errors in Functions Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2021-09-06 20:34 -0400
  Re: Detect Math Errors in Functions Juha Nieminen <nospam@thanks.invalid> - 2021-09-01 18:22 +0000
  Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-01 21:02 +0200
    Re: Detect Math Errors in Functions David Brown <david.brown@hesbynett.no> - 2021-09-02 11:53 +0200
  Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-01 21:10 +0200
    Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-02 05:31 +0200
      Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-02 07:51 +0200
    Re: Detect Math Errors in Functions Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2021-09-06 20:47 -0400
      Re: Detect Math Errors in Functions red floyd <no.spam.here@its.invalid> - 2021-09-06 20:24 -0700
        Re: Detect Math Errors in Functions Juha Nieminen <nospam@thanks.invalid> - 2021-09-08 10:20 +0000
          Re: Detect Math Errors in Functions David Brown <david.brown@hesbynett.no> - 2021-09-08 13:17 +0200
            Re: Detect Math Errors in Functions Hope Rouselle <hrouselle@jevedi.com> - 2021-09-08 10:24 -0300
      Re: Detect Math Errors in Functions Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-07 09:10 +0200

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