
Hello,
given a sparse matrix and an index i, what would be the most efficent way to set row i and column i to zero? Is there a better way than doing
int i = someIndex;
for (int j = 0; j < matrix.ColumnCount; j++)
{
matrix[j, i] = 0;
matrix[i, j] = 0;
}
And is it possible to get the iteration count when using iterative solvers?
Regards,
Chris



So I tried to be clever and did
var enumerator = sparse.IndexedEnumerator();
int i = someIndex;
foreach (var entry in enumerator)
{
// Set any entry in the ith row to zero.
if (entry.Item1 == i)
{
sparse[i, entry.Item2] = 0;
}
// Set any entry in the ith column to zero.
if (entry.Item2 == i)
{
sparse[entry.Item1, i] = 0;
}
}
but despite the fact that it isn't faster, the results differ from the first method (maybe I just did some stupid mistake, but I don't see). Here's the code:
private static void TestSetZero(Matrix<double> matrix)
{
int n = matrix.RowCount;
int step = 2; // Set every nth row/col to zero
List<int> indices = new List<int>(n / step);
for (int i = 0; i < n; i += step)
{
indices.Add(i);
}
SparseMatrix sparse1 = matrix.Clone() as SparseMatrix;
SparseMatrix sparse2 = matrix.Clone() as SparseMatrix;
if (sparse1 != null && sparse2 != null)
{
Console.WriteLine("Input matrix has " + sparse1.NonZerosCount + " nonzeros.");
Console.WriteLine("Setting every " + step + "th row/column to zero.");
// Begin test full
Stopwatch sw = new Stopwatch();
sw.Start();
foreach (var i in indices)
{
for (int j = 0; j < n; j++)
{
sparse1[j, i] = 0;
sparse1[i, j] = 0;
}
}
sw.Stop();
Console.WriteLine("Timing full: " + sw.ElapsedMilliseconds + "");
Console.WriteLine("Matrix has " + sparse1.NonZerosCount + " nonzeros.");
// Begin test sparse
sw.Restart();
var enumerator = sparse2.IndexedEnumerator();
foreach (var i in indices)
{
foreach (var entry in enumerator)
{
if (entry.Item1 == i)
{
sparse2[i, entry.Item2] = 0;
}
if (entry.Item2 == i)
{
sparse2[entry.Item1, i] = 0;
}
}
}
sw.Stop();
Console.WriteLine("Timing sparse: " + sw.ElapsedMilliseconds + "");
Console.WriteLine("Matrix has " + sparse2.NonZerosCount + " nonzeros.");
Console.WriteLine("Matrices equal? " + sparse1.Equals(sparse2) + "");
//Console.WriteLine("Matrices deep equal? " + DeepEqual(sparse1, sparse2) + "");
//DumpMatrix(sparse1, "sparse1.txt");
//DumpMatrix(sparse2, "sparse2.txt");
}
else
{
Console.WriteLine("Please load a sparse matrix.");
}
}
I used the sparsesmall.mat file for testing.


Nov 17, 2012 at 10:58 AM
Edited Nov 17, 2012 at 11:03 AM

BTW, I think
SparseMatrix sparse = (SparseMatrix)matrix;
should not throw an exception for a dense matrix, but instead return null. In the above code I use
SparseMatrix sparse = matrix as SparseMatrix;
which works fine.
EDIT: Though, of course, an excption while casting is more instructive than a null reference exception at a later point... don't know...


Nov 17, 2012 at 11:33 AM
Edited Nov 17, 2012 at 11:35 AM

I think the problem is that
if (entry.Item1 == i){sparse2[i, entry.Item2] = 0;} if (entry.Item2 == i){sparse2[entry.Item1, i] = 0;}
modify the data the enumerator is created over (i.e. the column indices and row pointers). Either MN should throw an exception here or the enumerator should be rewritten to handle modifying the internal storage while iterating over it (not sure that
is possible).
Regards
Marcus



Yes, you cannot modify any enumerable while enumerating over it, matrices are no difference.
To set a submatrix to zero, simply use matrix.Storage.Clear(rowIndex,rowCount,columnIndex,columnCount)
Btw, you can always directly access the CRS data from a spare matrix by casting it's .Storage to SparseCompressedRowMatrixStorage.
Thanks,
Christoph



Right. This works, but it's slow:
List<Tuple<int, int>> toZero = new List<Tuple<int, int>>(sparse.NonZerosCount);
var enumerator = sparse.IndexedEnumerator();
foreach (var i in indices)
{
foreach (var entry in enumerator)
{
if (entry.Item1 == i)
{
toZero.Add(new Tuple<int, int>(i, entry.Item2));
}
else if (entry.Item2 == i)
{
toZero.Add(new Tuple<int, int>(entry.Item1, i));
}
}
}
foreach (var item in toZero)
{
sparse[item.Item1, item.Item2] = 0;
}
So how do I use matrix.Storage.Clear? Tried the following, which doesn't work:
foreach (var i in indices)
{
// Set any entry in the ith row to zero.
sparse.Storage.Clear(i, 1, 0, n);
// Set any entry in the ith column to zero.
sparse.Storage.Clear(0, n, i, 1);
}



Actually, scratch Matrix.Storage.Clear on sparse matrices until the next release, I've noticed an issue that was missed by the unit tests.



FYI:
The issue I mentioned above has been fixed in the meantime. The next release of Math.NET Numerics will also add the following members to Matrix:
 ClearColumn(index)
 ClearRow(index)
 ClearSubMatrix(rowIndex,rowCount,columnIndex,columnCount)
Thanks,
Christoph



Thanks for your effort, this sounds great. I checked out the latest stuff, and there still seems to be a problem. Using direct indexing and the new clear methods give different results:
// List<int> indices = list of rows/columns to clear
foreach (var i in indices)
{
for (int j = 0; j < n; j++)
{
sparse1[j, i] = 0;
sparse1[i, j] = 0;
}
}
foreach (var i in indices)
{
sparse2.ClearRow(i);
sparse2.ClearColumn(i);
}
Console.WriteLine("Equal? " + sparse1.Equals(sparse2)); // Nope, no luck!
Again using the sparsesmall.mat file for testing.



Indeed, thanks for testing. I've converted the case to a unit test and took the chance to finally implement sparse submatrix clearing properly. As a side effect it is now also more efficient, especially for clearing rows.
Thanks,
Christoph

