Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #80944

Re: Detect Math Errors in Functions

Show all headers | View raw

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.

You would probably also want to use "long long" (or uint64_t), rather
than "long" (which is 32-bit on some systems), as you want big numbers.
 You might even want compiler-specific 128-bit types - after all, the
conjecture is known to be true up to at least 2 ^ 68.


The conjecture itself is rather interesting.  Here are a couple of
interesting videos on it (there are many, but these are from reliable
and interesting sources, to save you from dry lectures or trisectors who
say they have solved it):

<https://www.youtube.com/watch?v=094y1Z2wpJg>
<https://www.youtube.com/watch?v=5mFpVDpKX70>

Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web