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Groups > comp.lang.c > #138845
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: storing and retrieving a double may show a difference? |
| Date | 2018-12-21 11:25 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <877eg3m895.fsf@bsb.me.uk> (permalink) |
| References | <ae9f72d7-7b05-495c-a47a-80a9fd39332a@googlegroups.com> |
chanrasjid@gmail.com writes:
> All values are doubles.
Not, I hope, c and width_A.
> x = *(A + c * width_A + c);
It's more normal to write this as x = A[c * with_A + c], and in C99 you
can even write A[c][c] provided A is declared in the right way. (You
might want to ask about that once your current issue is resolved. C99
has a number of things that help when writing numerical programs.)
As to your issue... Are you sure width_A is correct and that A (and
diagonal_A) points to the right sized block of data? Those would be the
first places I'd look, particularly as the problem first shows up when
c==1. At the very least you need to post the declarations of these and
the code that "sets them up".
Can you, in fact, post the whole program?
<snip>
> But when the diagonal was stored much earlier, an after some blocks of
> codes, x and diagonal will fail the equality test.
>
> for (c = 0; c < width_A; ++c) {
> x = *(A + c * width_A + c);
> //diagonal_A[c] = x;
> if (x != diagonal_A[c]) {
> printf("c %d, x %.15g != diagonal %.15g\n", c, x, diagonal_A[c]);
> printf("diff %.15g\n", fabs(x - diagonal_A[c]));
> assert(0);
> }
>
> c 1, x 103997435021.915 != diagonal 103997435022.047
> diff 0.131515502929688
>
> Why a slight difference?
Rather than assert(0) when something is wrong, you can write
assert(A[c * width_A + c] == diagonal_A[c]);
at the top of the loop body and leave it there so as to catch any
recurrence of this issue, even after this problem is fixed.
--
Ben.
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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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