Re: storing and retrieving a double may show a difference?

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: storing and retrieving a double may show a difference?
Date	2018-12-22 11:06 +0000
Organization	A noiseless patient Spider
Message-ID	<875zvllt23.fsf@bsb.me.uk> (permalink)
References	(1 earlier) <499a90dd-d54d-4d5e-8357-3971b41bbc1c@googlegroups.com> <dfea3799-9803-4238-9596-4eb241454c31@googlegroups.com> <b63f9d6c-2e49-49ae-b97a-c1965bc9c6f9@googlegroups.com> <87pntulk9z.fsf@bsb.me.uk> <pvkgva$1rh$1@dont-email.me>

Show all headers | View raw

James Kuyper <jameskuyper@alumni.caltech.edu> writes:

> On 12/21/18 15:03, Ben Bacarisse wrote:
>> James Kuyper <jameskuyper@alumni.caltech.edu> writes:
>> <snip>
>>> The key thing seems to be the values of A[width_A+1] and
>>> diagonal[1]. If they start out equal (which should be tested),
>> 
>> They start out equal by inspection of the code!
>
> Yes, well one of the things I've learned is that when your code
> misbehaves, and you're not sure why, you shouldn't trust things that
> seem guaranteed to be true by "inspection of the code". Since the code
> is malfunctioning, at least one of those things isn't actually true.

I disagree.  I find that inspecting the code is often the simplest way
to find errors.  But even when it does not, it can save time by ruling
out (or at least putting to the bottom of your list) numerous
hypotheses.  Here, the diagonal element of the array and the element of
the 1D diagonal array are set from the same variable, x, on consecutive
lines.  Adding assert(a == b) after a = x; b = x; seems a waste of time
to me, even if a and b are more complex.  I might do that when I start
doubting everything, but it will be way down my list.

Now, I have come across compiler bugs that mean you can't believe what
you see in the code.  That leads you down into a rabbit hole where you
don't know what to believe, but fortunately, that is very rare.

<snip>
-- 
Ben.

Thread

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

csiph-web