tut01-page03-example02.cc (flens/examples/tut01-page03-example02.cc)

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#include <flens/flens.cxx>
#include <iostream>

using namespace flens;
using namespace std;

int
main()
{
///
/// So this is our plain C-array
///
    double data[4*4] = { 1,  2,  3,  4,
                         5,  6,  7,  8,
                         9, 10, 11, 12,
                        13, 14, 15, 16};

///
/// We define one typedef for a storage scheme that only references data
/// and another typedef for a corresponding general matrix type. The
/// trick is that you can construct `GeMatrix` objects from full storage
/// objects.  So what we later do is building a storage object that
/// just keeps internally a pointer to the C-array.
///
/// Note: The type `GeMatrixView` is actually identicall with the one
/// from the previous example (there we defined it via `GeMatrix::View`).
///
    typedef FullStorageView<double, ColMajor>  FSView;
    typedef GeMatrix<FSView>                   GeMatrixView;

///
/// We finally create a matrix view that references the C-array as follows:
///   - We wrap the C-array in an anonymous full storage view object
///   - and construct with it a general matrix view.
/// The syntax for the full storage view is
/// `FSView(numRows, numCols, data-pointer, leading dimension)`.
///
    GeMatrixView  A = FSView(4, 4, data, 4);

    cout << "A = " << A << endl;

///
/// With a little pointer arithmetic we also can create views for a
/// sub-matrix directly (note that `data+5`points to the element of
/// `A(2,3)`.
///
    GeMatrixView  B = FSView(3, 2, data+5, 4);

    cout << "B = " << B << endl;

///
/// Note that in the following the data of the C-array gets copied into
/// matrix `M`.  This is because `GeMatrixNoView` has a non-view storage
/// scheme.
///
    typedef GeMatrix<FullStorage<double, ColMajor> >  GeMatrixNoView;
    GeMatrixNoView  M = GeMatrixView(FSView(4, 4, data, 4));

///
/// Note that you only can construct a `GeMatrix` from a storage object
/// if the type of the storage object is identical with `GeMatrix::Engine`
/// (`Engine` is the storage scheme used by `GeMatrix`).  Hence, the
/// following would not compile:
///
//  GeMatrixNoView M = FSView(4, 4, data, 4);
//  error: This would not compile because
//         GeMatrixNoView::Engine is of type "FullStorage<...>"
//         FSView                 is of type "FullStorageView<...>"

///
/// Let us demonstrate what is a view and what not:
///   - We change `M(2,2)`,
///   - we change `B(1,2)`,
///   - we output `A`.
/// Only the change of `B(1,2)` affects elements of `A` as both matrices
/// share data.
///
    M(2,2) = -666;
    B(1,2) =  666;
    cout << "now: A = " << A << endl;
    cout << "Data dump of C-array: " << endl;
    for (int i=0; i<16; ++i) {
        cout << data[i] << "  ";
    }
    cout << endl;

    return 0;
}