On Thu, Sep 17, 2009 at 1:11 PM, Mark Johnson
<mj1@cog.brown.edu> wrote:
Thanks Jesse for your answer and the code fragment!
Is there a way of building a matrix or vector out of a pre-existing
array_type? I'd like to be able to view a matrix as a vector, and use
vector arithmetic on it.
What would be great would be a way of building a matrix or vector out of
any object v that implements (say) v.size() and v[index].
I also think it would be great to have "computed matrices", i.e., an
adaptor that constructs a matrix interface for an arbitrary function
object f that implements (say) f.size1(), f.size2() and f(i,j).
I'd be willing to implement such a thing, but I don't know whether the
returned object should be a matrix_expression, a matrix_reference or
whatever, and I don't know what things an object has to define in order
to be a vector or a matrix. If someone can point me to the appropriate
documentation that would be great!
Best,
Mark
On 01/-10/-28163 02:59 PM, Jesse Manning wrote:
For dense matrix types the underlying storage of the matrix is just an
array of m*n values. You can access this data directly from the matrix
and it will provide the "vector view" so to speak. I am not sure about
other types of matrices such as sparse or compressed since I have
mostly dealt with dense. Here is some example code:
ublas::matrix<int> mat1(3,3);
// fill matrix with values
mat1(0,0) = 1;
mat1(0,1) = 3;
mat1(0,2) = 2;
mat1(1,0) = 1;
mat1(1,1) = 0;
mat1(1,2) = 0;
mat1(2,0) = 1;
mat1(2,1) = 2;
mat1(2,2) = 2;
// get vector view of matrix elements
ublas::matrix<int>::array_type& myarray = mat1.data();
// print contents
std::cout<< "myarray size: "<< myarray.size()<< std::endl;
std::cout<< "[";
for (size_t i=0; i< myarray.size(); ++i)
{
std::cout<< myarray[i];
if (i != myarray.size()-1)
{
std::cout<< ",";
}
}
std::cout<< "]"<< std::endl;
// modify some contents of the matrix using the vector view
myarray[0] = 0;
myarray[1] = 0;
myarray[2] = 0;
// print contents to show changes
std::cout<< "myarray size: "<< myarray.size()<< std::endl;
std::cout<< "[";
for (size_t i=0; i< myarray.size(); ++i)
{
std::cout<< myarray[i];
if (i != myarray.size()-1)
{
std::cout<< ",";
}
}
std::cout<< "]"<< std::endl;