Re: improved QR & Cholesky updating

[John W. Eaton]

On 21-Jan-2009, Jaroslav Hajek wrote:

| I'm not really sure. The point is that in this way, you have, in an
| m-file, a clear indication that the qrupdate funcs are unavailable.

I'd rather not scatter a lot of these checks all around the sources
if we don't have to.  So what if it is suboptimal if the calculation
can be done and the code in the .m files is cleaner?  If you want the
better performance, then install the library that provides it.

| Sometimes there may be better workarounds. For instance, the fix I
| committed to fsolve is really simplistic; in fact, if qrupdate is not
| available, it is better to not form the qr factorization explicitly at
| all and simply pass the original jacobian to __dogleg__. This is
| something I want to do eventually.

Is it ever better to do that if qrupdate is available?  If so, then
I think this feature should be selected via an option, and not because
qrupdate is or is not available.

| I think this rule applies in general - IMHO, if a functionality is not
| available, it's better to disable a function than to provide a version
| that always fails, because the former condition can be checked
| gracefully and allows for possible workarounds.

Doesn't try/catch also allow you to perform a check for functionality
if you really must do it?

Maybe we should improve the error messages for the missing functions
as well.  Instead of just saying "foo: not available in this version
of Octave" we should say "foo: not available because libfoo was not
present when Octave was compiled".  Then maybe having the error makes
more sense, because it tells users why the function is missing.
Otherwise, they just think Octave sucks because it doesn't have the
function they are trying to use.

jwe