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Groups > comp.lang.c > #148781
| From | jak <nospam@please.ty> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: for-loop mutates to an endless-loop |
| Date | 2020-01-05 00:40 +0100 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <qur7pn$ij6$1@gioia.aioe.org> (permalink) |
| References | <h78q9lF3mhkU1@mid.individual.net> <87ftgwpqbi.fsf@bsb.me.uk> |
Il 03/01/2020 15:28, Ben Bacarisse ha scritto:
> "Walter H." <Walter.H-Nntp@mathemainzel.info> writes:
> <cut>
>> then I tried the same in C with several compilers
>> 32-bit, 64-bit Intel, GNU under Linux or Visual C++ under Windows
>> even QuickC or Turbo C every one built this programme with an endless-loop;
>>
>> [code]
>> #include <stdio.h>
>> int main( int argc, char* argv[ ] )
>> {
>> short int nIter;
>
> This may or may not by a 16-bit integer. It probably will be, but
> modern C has the type int16_t that you can use to be sure. #include
> <stdint.h> to get these new known-width types.
>
>> long lSum = 0;
>> for ( nIter = -32768; nIter <= 32767; nIter++ )
>
> When you need numbers like this it better to #include <limits.h> and use
> SHRT_MIN and SHRT_MAX. If you included stdint.h to use int16_t then
> INT16_MIN and INT16_MAX are available.
>
> Also, in modern C you can declare the loop variable in the for
> statement:
>
> for (short nIter = -32768; nIter <= 32767; nIter++) ...
>
>> lSum += nIter;
>> printf( "sum: %ld\n", lSum );
>> return 0;
>> }
>> [/code]
>
> Adding a bit to the answer you already have...
>
> If nInter is indeed a 16-bit integer type, and int is wider (the most
> likely situation here) then the behaviour of the code above is
> implementation-defined.
>
> Once nIter gets to 32767, the increment in the for loop causes a problem
> when the result (which will be of type int) is converted back to short.
> The result is either an implementation-defined value or an
> implementation-defined signal will be raised.
>
> Anyway, unless a signal is raised, the result is bound to be <= 32767
> so, as has been pointed out, the test is never false.
>
>> here the same in old FORTRAN which also worked as expected;
>>
>> [code]
>> PROGRAM SumDemo
>> INTEGER*2 nIter
>> INTEGER*4 lSum
>> lSum = 0
>> DO nIter = -32768,32767,1
>> lSum = lSum + nIter
>> END DO
>> WRITE ( *, * ) 'sum: ', lSum
>> END
>> [/code]
>
> Curiously, the GNU Fortran compiler gets this wrong (at least as far as
> my understanding of F77 goes) and generates an endless loop.
>
> <cut>
>> what would be the correct way of a for-loop
>> with the complete range of an integral type?
>
> One solution is to use a wider type:
>
> for (int_fast32_t i = INT16_MIN; i <= INT16_MAX; i++) ...
>
> but that is not always possible. The general solution is to test at the
> end, after the increment:
>
> short i = SHRT_MIN;
> do sum += i; while (i++ < SHRT_MAX);
>
> but this is not portable to implementations where (a) short is as wide
> as int (in which case the increment is undefined behaviour), or (b)
> implementations that raise a signal when converting an out-of-range
> value to short.
>
> A portable solution must increment only when i < SHRT_MAX:
>
> do sum += i; while (i < SHRT_MAX && i++ | 1);
>
> Of course you can just special-case the last value:
>
> for (i = SHRT_MIN; i < SHRT_MAX; i++)
> sum += i;
> sum += i;
>
> <cut>
>
...or just don't increment last time:
for (short int nIter = -32768;; nIter++)
{
lSum += nIter;
if(!(nIter < 32767))
break;
}
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for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-03 13:27 +0100
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-03 14:01 +0100
Re: for-loop mutates to an endless-loop Dan Purgert <dan@djph.net> - 2020-01-03 13:18 +0000
Re: for-loop mutates to an endless-loop James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-03 08:28 -0500
Re: for-loop mutates to an endless-loop Dan Purgert <dan@djph.net> - 2020-01-03 13:33 +0000
Re: for-loop mutates to an endless-loop James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-03 21:10 -0500
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-03 15:34 +0100
Re: for-loop mutates to an endless-loop James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-03 08:26 -0500
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-04 16:15 +0100
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-04 17:17 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 16:20 +0000
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-04 18:43 +0100
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-04 19:28 +0100
Re: for-loop mutates to an endless-loop Bart <bc@freeuk.com> - 2020-01-04 18:59 +0000
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-05 14:24 +0100
Re: for-loop mutates to an endless-loop Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-01-04 13:47 -0800
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-05 14:26 +0100
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-06 20:31 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 22:25 +0000
Re: for-loop mutates to an endless-loop Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-01-04 14:08 -0800
Re: for-loop mutates to an endless-loop James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-04 21:32 -0500
Re: for-loop mutates to an endless-loop scott@slp53.sl.home (Scott Lurndal) - 2020-01-05 21:48 +0000
Re: for-loop mutates to an endless-loop James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-04 21:43 -0500
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 14:47 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 14:39 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 15:42 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 15:07 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 16:18 +0100
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 16:47 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 20:41 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:05 +0100
Re: for-loop mutates to an endless-loop gazelle@shell.xmission.com (Kenny McCormack) - 2020-01-03 18:30 +0000
Re: for-loop mutates to an endless-loop Robert Wessel <robertwessel2@yahoo.com> - 2020-01-04 00:23 -0600
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:10 +0100
Re: for-loop mutates to an endless-loop scott@slp53.sl.home (Scott Lurndal) - 2020-01-04 16:41 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 17:44 +0100
Re: for-loop mutates to an endless-loop Bart <bc@freeuk.com> - 2020-01-04 11:15 +0000
Re: for-loop mutates to an endless-loop Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-04 12:33 +0000
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-04 15:59 +0100
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 16:01 +0100
Re: for-loop mutates to an endless-loop Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-04 16:04 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 17:06 +0100
Re: for-loop mutates to an endless-loop scott@slp53.sl.home (Scott Lurndal) - 2020-01-04 16:42 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 17:44 +0100
Re: for-loop mutates to an endless-loop Robert Wessel <robertwessel2@yahoo.com> - 2020-01-04 17:55 -0600
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-06 20:43 +0100
Re: for-loop mutates to an endless-loop scott@slp53.sl.home (Scott Lurndal) - 2020-01-06 19:50 +0000
Re: for-loop mutates to an endless-loop Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-01-06 15:37 -0500
Re: for-loop mutates to an endless-loop scott@slp53.sl.home (Scott Lurndal) - 2020-01-06 21:59 +0000
Re: for-loop mutates to an endless-loop Robert Wessel <robertwessel2@yahoo.com> - 2020-01-06 17:33 -0600
Re: for-loop mutates to an endless-loop gazelle@shell.xmission.com (Kenny McCormack) - 2020-01-03 18:31 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:06 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 01:38 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:10 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 16:39 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 17:41 +0100
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 17:49 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 22:49 +0000
Re: for-loop mutates to an endless-loop Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-05 03:44 +0100
Re: for-loop mutates to an endless-loop David Brown <david.brown@hesbynett.no> - 2020-01-04 13:09 +0100
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-03 14:28 +0000
Re: for-loop mutates to an endless-loop jak <nospam@please.ty> - 2020-01-05 00:40 +0100
Re: for-loop mutates to an endless-loop "Walter H." <Walter.H-Nntp@mathemainzel.info> - 2020-01-06 20:50 +0100
Re: for-loop mutates to an endless-loop Manfred <noname@add.invalid> - 2020-01-06 21:28 +0100
Re: for-loop mutates to an endless-loop Manfred <noname@add.invalid> - 2020-01-06 21:02 +0100
Re: for-loop mutates to an endless-loop Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 10:01 -0800
Re: for-loop mutates to an endless-loop Richard Damon <Richard@Damon-Family.org> - 2020-01-04 16:21 -0500
Re: for-loop mutates to an endless-loop Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-01-04 22:17 +0000
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