[Cbc] Regarding matrix operations within coinor

[Ashutosh Mahajan]

On Mon, Jul 21, 2014 at 4:55 PM, G Chandramouli <gcmouli1 at gmail.com> wrote:
> Dear all,
>          In CoinPackedMatrix(or other coinor library)  is there
>  a way to get the matrix in some standard format (for e.g. CSC etc),
> so that I can use some  standard libraries like armadillo or eigen.

CoinPackedMatrix is already in CSC or CSR format, you can easily get
the three vectors (values, indices and starting-positions) as follows:

const double *vals = cmat->getElements();
const int *cols = cmat->getIndices();
const int *row_starts = cmat->getVectorStarts();
(assuming that the cmat is a row-ordered CoinPackedMatrix)


>  ...
>
>  Also, is there is a way to create identity matrix outside
> (let us say using some 2d array in C++) and then import it to a
>  matrix in CoinPackedMatrix form. Doing it directly, takes a lot of
>  time when no. of columns is large. Is there a better way to do this.
>
> I did the following stuff, but it took nearly 700 secs for
>  numberColumns >200000
>
>  ------------------------------------------------------------------
> CoinPackedMatrix *matrix_col=new CoinPackedMatrix(false,0,0);
> matrix_col->setDimensions(0,numberColumns);
> CoinPackedVector *row=new CoinPackedVector[numberColumns];
> for(int i=0;i<numberColumns;i++)
>         {
>                 row[i].insert(i,1);
>                 matrix_col->appendRow(row[i]);
>         }
> -------------------------------------------------------------------

appendRow() and appendCol() are usually slow. Here's an alternate code
for the same matrix (i.e. an identity matrix):

  int numberColumns = 100000;
  int *rowIndices = new int[numberColumns];
  int *colIndices = new int[numberColumns];
  double *elements = new double[numberColumns];

  for (int i=0; i<numberColumns; ++i) {
    rowIndices[i] = i;
    colIndices[i] = i;
    elements[i] = 1.0;
  }
  CoinPackedMatrix *matrix_col=new CoinPackedMatrix(false, rowIndices,
                                                    colIndices, elements,
                                                    numberColumns);

  std::cout << "size of A = " <<  matrix_col->getNumElements();


Cheers
Ashutosh