/// <summary>
/// Returns a sparse sub-matrix containing the requested rows.
/// </summary>
/// <param name="matrix"></param>
/// <param name="rowIndices">The rows to extract.</param>
/// <returns>The requested sub-matrix.</returns>
public static SparseMatrix FastGetRows(this SparseMatrix matrix, int[] rowIndices)
{
var target = new SparseMatrix(rowIndices.Length, matrix.ColumnCount);
matrix.FastGetRows(rowIndices, target);
return(target);
}