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Groups > comp.lang.c > #148774
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: for-loop mutates to an endless-loop |
| Date | 2020-01-04 22:17 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <87v9pqn9xy.fsf@bsb.me.uk> (permalink) |
| References | <h78q9lF3mhkU1@mid.individual.net> <Hl7QF.26306$gQ2.20665@fx25.iad> |
Richard Damon <Richard@Damon-Family.org> writes:
> I will point that there is a fundamental limitation in the for loop,
> that when running on cycling the value of a variable, there MUST be
> some value that won't be used in the loop, to use for the exit
> case. Since the test clause is run BEFORE each loop, if it is testing
> the value of the loop variable, there needs to be a value that the
> loop won't run on to test for and exit the loop.
>
> I know of two options here, one is to just use a wide variable for the
> loop, so if you want all value of an int to be used, then you need to
> choose a type for the loop counter that is bigger than an int.
>
> A second option is to switch from a for loop to a do while of the form:
>
> var = start value for loop
> do {
> // Code for loop
> if(var == max value for loop) break
> var++
> } while (1);
There is a "tricksy" partial for-based solution as well. If the loop
body is made up of expression statements, or you can re-write it as a
function, you can write
for (T i = min-value-of-T; f(... i ...), i < max-value-of-T; i++);
In the case in question one can write:
long sum = 0;
for (short i = SHRT_MIN; sum += i, i < SHRT_MAX; i++);
printf("%ld\n", sum);
The idea cones form Algol 68 where every expression can be a
value-yielding block. This gave one a way to do exit in the middle
loops without break by writing part of the loop body in the while
condition.
If you have block expressions (as gcc provides) this is in fact a
general solution, though it probably still rates as yucky for normal
programmers.
> IF (and only if) the compiler promises not to optimize on the
> undefined behavior, and the actual undefined behavior of increment is
> benign, then (and only then) you can combine the test and the
> increment into the while clause, to get something like:
>
> var = start value for loop
> do {
> // Code for loop
> } while(var++ != max value for loop);
>
> Note, by checking the value before the increment we can test for the
> last value, but because the increment will still happen, the undefined
> behavior of the overflow does happen, so it must be benign and not
> optimized against.
Yup, for signed types no narrower than int. As I am sure you know,
there will be no UB if var is unsigned, and if var has a type narrower
than int the behaviour is implementation defined.
--
Ben.
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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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