Hi.

To answer your question. Both of your ways are wrong, but the second is not beyond hope :). You can't implement this with BOOST_FOREACH because a matrix is not a container. The solution from Joaquim will of course also work and perform ssentially the same operations, however i am not so sure about the costs :).

boost::numeric::ublas::compressed_matrix<u32>::iterator1 i=M.begin1();
//important: store end iterator here, this is super expensive to construct

boost::numeric::ublas::compressed_matrix<u32>::iterator1 iEnd = M.end1();
//iterate over the rows

for (; i != iEnd; ++i)

{
//use dual iterator to iterate over the columns of the current row. again: this is expensive to construct.

boost::numeric::ublas::compressed_matrix<u32>::iterator2 j = i.begin();
boost::numeric::ublas::compressed_matrix<u32>::iterator2 jend = i.end();

for (; j != jend; ++j)

{

// use j.index1(), j.index2(), *j to access each non zero element of M

*j = v(j.index1()); // do something with each non zero element of both matrix and vector…

} // end for

} // end for

depending on what you exactly want to do there might be a lot faster ways (factor 3+). This is the way that works with every matrix/matrix expression. if you are only interested in the column index and column value of the compressed_matrix you can have direct access to the internal data structures.

On 04.10.2013 23:19, Reich, Darrell wrote:

The two pieces of code below attempt to iterate over the non zero elements of a matrix, both crash. I suspect from looking over the various discussions online that the correct placement of & is critical? Assuming it is possible to create two working examples of BOOST_FOREACH matrix and matrix iterators, which method is preferred / “better” / more efficient? Thanks!

#include <boost/foreach.hpp>

#include <boost/numeric/ublas/matrix.hpp>

#include <boost/numeric/ublas/matrix_sparse.hpp>

#include <boost/numeric/ublas/matrix_expression.hpp>

#include <boost/numeric/ublas/matrix_proxy.hpp>

#include <boost/numeric/ublas/vector.hpp>

#include <boost/numeric/ublas/vector_sparse.hpp>

#include <boost/numeric/ublas/vector_expression.hpp>

#include <boost/numeric/ublas/vector_proxy.hpp>

boost::numeric::ublas::compressed_matrix<float> M(n,n);

boost::numeric::ublas::compressed_vector<float> v(n);

// Example 1: BOOST for each…

// TODO: 2D nested matrix example?

BOOST_FOREACH(float, v)

{

   v += 1; // do something with each non zero element of v…

} // end for each

// Example 2: iterators

boost::numeric::ublas::compressed_matrix<u32>::iterator1 i;

boost::numeric::ublas::compressed_matrix<u32>::iterator2 j;

for (i = M.begin1(); i != M.end1(); ++i)

{

   for (j = M.begin2(); j != M.end2(); ++j)

   {

      // use j.index1(), j.index2(), *j to access each non zero element of M

      *j = v(j.index1()); // do something with each non zero element of both matrix and vector…

   } // end for

} // end for

Extra credit: I suspect I should protect the code with some kind of assert to ensure the size of vector is the same as the size of matrix.
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