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