Hi,
So I spent some time today investigating a 'bug'. The prototype for nmf is:
template <typename ScalarType>
void nmf(viennacl::matrix<ScalarType> const & V,
viennacl::matrix<ScalarType> & W,
viennacl::matrix<ScalarType> & H,
nmf_config const & conf)
Now, if W or H is a zero matrix than prod(W, H) is also a zero
matrix. So prod(W, H) - V = -V. And ||V|| = ||-V||, so the difference
between the initial norm and the norm after the 1st iteration is 0,
which is always less than conf.tolerance(). So the routine quits.
I think this is a bug in the algorithm: why should the behaviour depend
on the initial state of W and H, unless the initial W and H are supposed
explicitly to be taken as approximations? But if so, I don't think a
zero matrix approximation should cause the algorithm to quit!
The following change is sufficient to fix this. I've committed it and a
change ensuring any ViennaCL objects created during the run are in the
right context. I don't know if it is ideal, since it means that in the
(rare) case where W and H in their initial state are adequate, two sets
of check_after_steps iterations are computed, where previously only one
set would have been. There are alternatives: of course, we could compute
norm_frobenius(prod(W, H)) and check that it is not 0 on the first
iteration, but that's much more expensive in every other case.
diff --git a/viennacl/linalg/nmf.hpp b/viennacl/linalg/nmf.hpp
index e47712d..02e4007 100644
--- a/viennacl/linalg/nmf.hpp
+++ b/viennacl/linalg/nmf.hpp
@@ -173,7 +173,7 @@ namespace viennacl
std::cout << diff_val / diff_init << std::endl;
// Approximation check
- if (diff_val / diff_init < conf.tolerance())
+ if ((diff_val / diff_init < conf.tolerance()) && (conf.iters_ !=
conf.check_after_steps() + 1))
break;
// Stagnation check
Cheers,
Toby
--
Toby St Clere Smithe
http://tsmithe.net
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