Re: [boost] Boost.Numeric: multi_array spec/docs

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-04-03 00:24:53

On Mon, 2 Apr 2001, Larry Evans wrote:
jcampb> 
jcampb> One example of subarray slicing is the recursive strassen[1] matrix multiplication.
jcampb> However, I've also heard that this is NOT used in practice because it's slow
jcampb> (I guess because of the recursion).

There have been a few cases where people have used strassen. At one point
Cray used it in there implementation the BLAS gdemm to get a better
linpack benchmark result. One trouble with strassen is that it is only
better than the usual algorithm when the matrices get large. The typical
approach is to do strassen-style recursion until the submatrices are small
enough, and then use the normal algorithm.

jcampb> I'm unsure what indirect slicing is.  Is it the use of an vector of indices to create
jcampb> a subarray, as in the following?
jcampb> 
jcampb>    int indices0[] = {1,5,6,10};
jcampb>    int indices1[] = {5,4,6};
jcampb>    int xshape[] = {10,6};
jcampb>    array<double,2> x(xshape);
jcampb>    int yshape[] = {4,3};
jcampb>    array<double,2> y(yshape);
jcampb>    y = x[indices0][indices1];

Yep, that's it. Also, see the example in the multi_array docs Ron and I
are working on:

http://www.lsc.nd.edu/~jsiek/tlc/multi_array/libs/numeric/doc/multi_array.html

----------------------------------------------------------------------
 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate                  email: jsiek_at_[hidden]
 Univ. of Notre Dame         work phone: [phone]
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