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Groups > comp.lang.c > #138988
| Subject | Re: A final word on casting. |
|---|---|
| Newsgroups | comp.lang.c |
| References | (2 earlier) <87k1k0ie66.fsf@bsb.me.uk> <NaNTD.109597$krL1.91@fx15.am4> <875zvki950.fsf@bsb.me.uk> <fabc96a8-762e-430e-a567-8658f60fe0d5@googlegroups.com> <QdOTD.34006$Ck1.30728@fx18.am4> |
| From | Richard Damon <Richard@Damon-Family.org> |
| Message-ID | <pJRTD.94984$gU6.40605@fx33.iad> (permalink) |
| Organization | Forte - www.forteinc.com |
| Date | 2018-12-23 14:53 -0500 |
On 12/23/18 10:55 AM, Bart wrote:
> On 23/12/2018 15:08, chanrasjid@gmail.com wrote:
>> On Sunday, December 23, 2018 at 10:57:14 PM UTC+8, Ben Bacarisse wrote:
>>> Bart <bc@freeuk.com> writes:
>>>
>>>> On 23/12/2018 13:08, Ben Bacarisse wrote:
>>>>> Bart <bc@freeuk.com> writes:
>>>>
>>>>>> Try:
>>>>>>
>>>>>> printf("%d\n", (int)1.9999999999999999999999999999999);
>>>>>>
>>>>>> I seem to get the answer 2.
>>>>>
>>>>> Are you really puzzled by that?
>>>>
>>>> Actually, yes. It means that (int)1.something will usually be 1 but
>>>> sometimes 2.
>>>
>>> At least you won't be puzzled any more.
>
> Don't be so sure.
>
>> I think it is reasonable. C gives the closest double to represent
>> 1.999... and it is 2.0; So (int) x gives the integral part is "almost"
>> official.
>
> How does "1.999999999999..." in source code end up as /exactly/ 2.0?
>
> Try this program, which starts from 1.0 then successively ends 0.9,
> 0.09, 0.009 etc to get progressively longer versions of 1.999...
>
> It gets closer and closer to 2.0, but is never exactly 2.0:
>
> #include <stdio.h>
>
> int main (void) {
> double a,b;
>
> a=1.0;
> b=0.9;
> for (int i=0; i<30; ++i) {
> a+=b;
> printf("a: %.24f, 2.0-a: %.10e, a%s2.0, (int)a: %d, b: %e\n",
> a, 2.0-a, (a==2.0?"==":"!="), (int)a, b);
> b/=10.0;
> }
> }
>
> So it appears to be either rounding deliberately applied by a C
> compiler, or a side-effect of how the calculation is actually done.
>
One issue is that by the standard, to achieve the specified accuracy,
the implementation isn't allowed to just simply get the value of the
places with a chain of multiplication by 0.1 (or division by 10.0), as
that may build up too much accumulated error (and the difference between
2.0 and the number just below it is a very small number, so tiny errors
might be important).
Instead I would expect something more like:
int main(void) {
double a, b, c;
a = 1.0;
b = 1.0;
for(int i = 0; i < 30; i++) {
a = 10 * a + 9;
b = 10 * b;
c = a/b;
printf("c: %.24f, 2.0-c: %.10e, c$s2.0, (int)c: %d b:%e\n",
c, 2.0-c, (c == 2.0) ? "==" : "!=", (int) c, b);
}
}
Since a and b will always hold an 'integral' value, until their
magnitude exceeds the precision that can be stored in a double, the only
round off will be in computing a/b
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A final word on casting. chanrasjid@gmail.com - 2018-12-23 03:02 -0800
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 12:11 +0000
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 13:08 +0000
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 14:43 +0000
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 14:56 +0000
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-23 07:08 -0800
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 15:55 +0000
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 16:07 +0000
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 08:21 -0800
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 17:56 +0000
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 19:23 +0000
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 20:12 +0000
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 14:06 -0800
Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:34 -0800
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-28 20:29 +0000
Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 15:07 -0800
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-28 20:56 -0500
Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-29 09:22 -0800
Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2019-01-01 13:27 -0800
Re: A final word on casting. Bart <bc@freeuk.com> - 2019-01-02 01:07 +0000
Re: A final word on casting. Bart <bc@freeuk.com> - 2019-01-03 18:55 +0000
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2019-01-03 22:27 -0500
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-01-04 16:25 -0500
Re: A final word on casting. Ike Naar <ike@iceland.freeshell.org> - 2018-12-28 19:10 +0000
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-28 12:33 -0800
Re: A final word on casting. "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-23 14:11 -0800
Re: A final word on casting. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-12-24 02:54 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:24 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:29 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:45 -0800
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-24 13:44 +0000
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-24 08:51 -0500
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-23 14:53 -0500
Re: A final word on casting. Robert Wessel <robertwessel2@yahoo.com> - 2018-12-24 09:39 -0600
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-24 13:38 -0500
Re: A final word on casting. Manfred <noname@add.invalid> - 2018-12-23 21:22 +0100
Re: A final word on casting. Manfred <noname@add.invalid> - 2018-12-23 21:33 +0100
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 10:56 -0500
Re: A final word on casting. Sjouke Burry <burrynulnulfour@ppllaanneett.nnll> - 2018-12-23 17:18 +0100
Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 20:03 +0000
Re: A final word on casting. "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-28 14:47 -0500
Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 12:24 +0000
Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-23 07:28 -0500
Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 07:50 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:35 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:42 -0800
Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:52 -0800
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 10:28 -0500
Re: A final word on casting. Barry Schwarz <schwarzb@dqel.com> - 2018-12-23 11:39 -0800
Re: A final word on casting. Ike Naar <ike@iceland.freeshell.org> - 2018-12-24 07:56 +0000
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 14:03 -0800
Re: A final word on casting. Barry Schwarz <schwarzb@dqel.com> - 2018-12-24 14:39 -0800
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 14:50 -0800
Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 15:34 -0800
Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 15:23 -0800
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 20:34 -0800
Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 14:52 -0800
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 20:25 -0800
Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-24 17:52 -0500
Re: A final word on casting. luser droog <luser.droog@gmail.com> - 2018-12-24 21:27 -0800
Re: A final word on casting. luser droog <luser.droog@gmail.com> - 2018-12-24 21:39 -0800
Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 22:36 -0800
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