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Groups > comp.lang.c > #138918
| From | Manfred <noname@add.invalid> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: storing and retrieving a double may show a difference? |
| Date | 2018-12-22 21:41 +0100 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <pvm7h1$hga$1@gioia.aioe.org> (permalink) |
| References | <ae9f72d7-7b05-495c-a47a-80a9fd39332a@googlegroups.com> <499a90dd-d54d-4d5e-8357-3971b41bbc1c@googlegroups.com> <dfea3799-9803-4238-9596-4eb241454c31@googlegroups.com> <87va3mls9q.fsf@bsb.me.uk> |
On 12/21/2018 6:10 PM, Ben Bacarisse wrote:
> chanrasjid@gmail.com writes:
> <snip>
>> int width_A, height_A;// compute earlier
>>
>>
>> // gaussian elimination for MLR, num_war,etc...
>>
>> i 1, x 158161154320.539 != diagonal 158161154320.547
>> test: mlr-v1.c:561: gauss: Assertion `0' failed.
>>
>> I think the first answer abou fpu 80 bits and ram 64 bits may be the
>> reason.
>
> No, it's a straight forward logic error. You change A[1][1] after
> calculating diagonal[1] so when you compare the two, they are no longer
> the same.
Besides, also by simple code inspection, there appears to be more to be
corrected - error propagation is not correctly estimated, which affects
the assertions. More precisely at least two ingredients of this are missing:
- using DBL_EPSILON
- considering how many operations are involved in the checked result
that can affect accuracy (esp. additions and subtractions)
I have marked two examples below
>
> I hate to say it, but you are very unlikely to get sound advice from fir.
>
> I've marked the line where it happens:
>
<snip>
>> //reduce UT upper triangle to 0
>> for (i = 0; i < width_A; ++i) {
>> // div col i by leading non-zero element == diag element a-ii
>> x = *(A + i * width_A + i);
>> //diagonal
/****
Ben is right that there is a logic error, but still in floating point ==
and != tests require extra attention: in fact they are more correctly
used as /identity/ tests rather than /compare equal/ tests.
Here this is correct only for architectures that are guaranteed to
preserve the FP binary representation.
****/
>> if (i > 0 && x != diagonal[i]){
>> printf("i %d, x %.15g != diagonal %.15g\n",i, x ,diagonal[i]);
>> assert(0);
>> }
>> for (j = i; j < height_A; ++j) {
>> *(A + j * width_A + i) /= x;
>> }
>> assert(*(A + i * width_A + i) == 1.0);
>> // now diag elem = 1;
>> for (k = i + 1; k < width_A; ++k) {
>> x = *(A + i * width_A + k); // mult
>> for (j = i; j < height_A; ++j) {
>> *(A + j * width_A + k) -= x * *(A + j * width_A + i);
>
> **** Here you can have k == 1 and j == 1 and you change the element of A
> that is on the diagonal.
>
>> }
/*****************
This is another example:
- The constant 1.0e-300 is arbitrary. The test should be based on
DBL_EPSILON an (some estimate of) the size of x.
- Is this meant to be a test like == 0 or < 0 ? If it is the former,
then a fabs() call is missing
******************/
>> assert(*(A + i * width_A + k) < 1.0e-300);
>> }
>> }
>
> <snip rest of function>
>> return x;
>> }
>
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storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-20 23:25 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 00:56 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 07:02 -0800
Re: storing and retrieving a double may show a difference? Bart <bc@freeuk.com> - 2018-12-21 15:33 +0000
Re: storing and retrieving a double may show a difference? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-21 11:16 -0500
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 09:05 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:43 -0800
Re: storing and retrieving a double may show a difference? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-21 12:57 -0500
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 20:06 +0000
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 17:10 +0000
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:17 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:20 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:24 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 21:46 -0800
Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-22 21:41 +0100
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-22 21:08 +0000
Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-23 20:34 +0100
Re: storing and retrieving a double may show a difference? Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:14 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 19:16 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:12 -0800
Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 09:23 -0800
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 20:03 +0000
Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-22 00:10 -0500
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-22 11:06 +0000
Re: storing and retrieving a double may show a difference? mark.bluemel@gmail.com - 2018-12-21 01:00 -0800
What type of problem is it? (Was: storing and retrieving a double may show a difference?) gazelle@shell.xmission.com (Kenny McCormack) - 2018-12-21 09:10 +0000
Re: What type of problem is it? (Was: storing and retrieving a double may show a difference?) mark.bluemel@gmail.com - 2018-12-21 01:33 -0800
Re: storing and retrieving a double may show a difference? Bart <bc@freeuk.com> - 2018-12-21 10:16 +0000
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 11:25 +0000
Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 08:06 -0500
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 07:13 -0800
Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 09:10 -0800
Re: storing and retrieving a double may show a difference? Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 08:16 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 08:09 -0800
Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 17:23 -0800
Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 17:58 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 19:30 -0800
Re: storing and retrieving a double may show a difference? Richard Damon <Richard@Damon-Family.org> - 2018-12-22 22:48 -0500
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 20:10 -0800
Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 20:41 -0800
Re: storing and retrieving a double may show a difference? Barry Schwarz <schwarzb@dqel.com> - 2018-12-22 22:02 -0800
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-23 02:43 -0800
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 12:38 +0000
Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-23 05:12 -0800
Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-23 05:32 -0800
Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 14:55 +0000
Re: storing and retrieving a double may show a difference? Richard Damon <Richard@Damon-Family.org> - 2018-12-23 07:43 -0500
Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-23 20:42 +0100
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