Using CXXBLAS

FLENS matrix/vector types make iteraction with BLAS easy as you have access to

matrix/vector dimensions,
storage order,
leading dimensions and strides and
data pointers.

In addition CXXBLAS makes it easy to reuse the same code for different element types like float, double, std::complex<float>, std::complex<double> or any other type that “can be used like a double”, i.e. relevant operators (e.g. +, *, ...) are overloaded.

Toy Example

Assume you want to compute \(y = \beta y + \alpha\,A^T x + z\) where \(\alpha, \beta\) are scalars, \(x, y, z\) are (dense) vectors and \(A\) is a (general) matrix (with full storage).

Using CXXBLAS for this task you will first call gemv and subsequently axpy.

Example Code

flens/examples/tut02-page01-example.cc

#include <cxxstd/iostream.h>

#include <flens/flens.cxx>

using namespace flens;

using namespace std;

int

main()

{

    typedef double                               T;

    typedef DenseVector<Array<T> >               DEVector;

    typedef GeMatrix<FullStorage<T, ColMajor> >  GEMatrix;

    const T  alpha = 1.5,

             beta = 2.5;

    DEVector x(3), y(3), z(3);

    x = 1, 2, 3;

    y = 2, 3, 4;

    z = 3, 4, 5;

    GEMatrix A(3,3);

    A = 1, 2, 3,

        5, 6, 7,

        5, 4, 3;

    cxxblas::gemv(A.numRows(), A.numCols(),

                  alpha,

                  true, false, A.data(), A.strideRow(), A.strideCol(),

                  x.data(), x.stride(),

                  beta,

                  y.data(), y.stride());

    cxxblas::axpy(y.length(), T(1), z.data(), z.stride(), y.data(), y.stride());

    cout << "y = " << y << endl;

    return 0;

}

Comments on Example Code

Compute \(y = \beta y + \alpha A^T x\)

    cxxblas::gemv(A.numRows(), A.numCols(),

                  alpha,

                  true, false, A.data(), A.strideRow(), A.strideCol(),

                  x.data(), x.stride(),

                  beta,

                  y.data(), y.stride());

Compute the update \(y = y + z\)

cxxblas::axpy(y.length(), T(1), z.data(), z.stride(), y.data(), y.stride());

Compile and Run

$shell> cd flens/examples                                                         
$shell> g++ -Wall -std=c++11 -I../.. tut02-page01-example.cc                      
$shell> ./a.out                                                                   
y = 
           47           50.5             54