[Jprogramming] Problem with sparse tridiagonal solver

[Henry Rich]

Not a problem for the guy who doesn't have to code it! :)  It would 
require row and column exchanges to change the pivot, which I'm guessing 
is more work than Roger wanted to put into it.

But it IS misdiagnosed: domain error should mean the input is invalid, 
while nonce error means the input is valid but unsupported by the 
current code.

Henry Rich

On 7/27/2018 3:08 PM, Bill Heagy wrote:
> Sorry, when I said "misdiagnosed", I meant that it was detecting a 
> problem that wasn't actually there, not giving the wrong message. 
> Starting with a zero shouldn't be a problem.
>
> On 2018-07-27 02:57 PM, Henry Rich wrote:
>> You're right that it should be diagnosed as a nonce error rather than 
>> a domain error.  If you try to invert a sparse matrix with a 0 in the 
>> top-left, you get a nonce error.  The code just doesn't handle the 
>> case of starting with a 0.
>>
>> I would change the error to nonce error, but that doesn't fix the 
>> testcase, and I don't think it's worth any effort to fix this bug in 
>> the solver.
>>
>> Henry Rich
>>
>> On 7/27/2018 2:49 PM, Bill Heagy wrote:
>>> Most of the time, the test works.  It's only if you run it 
>>> repeatedly, until there's a (random) zero in the (0,0) position, 
>>> that it fails. It looks like a misdiagnosed problem, or a problem 
>>> with initializing the solution.
>>>
>>> On 2018-07-27 02:43 PM, Raul Miller wrote:
>>>> Which suggests that maybe this is diagnosing an api handling issue?
>>>>
>>>> (If the test used to work, x and y somehow got flipped?)
>>>>
>>>> Thanks,
>>>>
>>> On Fri, Jul 27, 2018 at 2:37 PM Bill Heagy <wheagy at bell.net> wrote:
>>> >
>>> > If you just take the x and y and flip them so the 0 is in the bottom
>>> > right, it appears to work.  It fits the domain.
>>> >
>>> > On 2018-07-27 02:24 PM, Henry Rich wrote:
>>> >> Right again.  The sparse matrix divide is very restricted in 
>>> domain and
>>> >> I am not going to be the one to try to improve it. Maybe I can 
>>> tweak the
>>> >> testcase.
>>> >>
>>> >> Henry Rich
>>> >>
>>> >> On 7/27/2018 1:48 PM, Bill Heagy wrote:
>>> >>> Running gsp1 repeatedly produces:
>>> >>>
>>> >>> ...
>>> >>>    1e_13 >. >./ | (y=: h 11) (%. -: (%.$.)) x=: 0.01*g 11
>>> >>> |domain error
>>> >>> |   1e_13>.>./|(y=:h 11) (%.-:(%.$.))x=:0.01*g 11
>>> >>> |[-37] /home/wheagy/git/jsource/test/gsp1.ijs
>>> >>>
>>> >>> at various locations.
>>> >>> The problem appears to be a "0" in the (0,0) position in the x 
>>> matrix.
>>> >>>    JVERSION         Engine: j806/j32/linux
>>> >>> Beta: GPL3/2018-07-25T05:05:57
>>> >>> Library: 8.05.14
>>> >>> Platform: Linux 32
>>> >>> Installer: unknown
>>> >>> InstallPath: /home/wheagy/tmp/jbld/j32
>>> >>> Contact: wheagy at bell.net
>>> >>> 
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