Direct Access to CvMat

If CV_32FC1 (1 channel float)

CvMat* M = cvCreateMat( 4, 4, CV_32FC1 );
M->data.fl[ row * M->cols + col ] = (float)3.0;

or...

cvmGet( mat, row, col );

...


inline void cvDoubleMatPrint( const CvMat* mat )
{
    int i, j;
    for( i = 0; i < mat->rows; i++ )
    {
        for( j = 0; j < mat->cols; j++ )
        {
            printf( "%f ",cvmGet( mat, i, j ) );
        }
        printf( "\n" );
    }
}

from http://note.sonots.com/OpenCV/MatrixOperations.html