Setting elements to zero in CRSMatrix

[rabbanyk]

Hi there,

I was trying to set some elements in a Sparse Matrix (CRSMatrix) to zero, using nonZeroIterator, or apply procedure, and it seems to mess with the matrix and cause non valid results for further operations, in my case subtract. To be more specific, I'm extending the CRS with the following method:

public void zeroDiagonalsInPlace() {
        MatrixIterator it = nonZeroIterator();
        while (it.hasNext()) {
            double x= it.next();
            if (it.rowIndex() == it.columnIndex()) 
                        it.set(0);
        }
    }
or 
    MatrixProcedure procedure = new MatrixProcedure() {
            @Override
            public void apply(int i, int j, double value) {
                if (i==j)
                    set(i,j,0);
            }
        };
         this.eachNonZero(procedure );

The method seems to do the job but then I am subtracting another matrix from this resulted matrix, and the results are wrong. See example bellow, A = 1.000 0.000 0.000 1.000 1.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 1.000 0.000 0.000 1.000 1.000 1.000 0.000 0.000 1.000 1.000

B = 1.000 0.000 0.000 1.000 1.000 0.000 1.000 1.000 1.000 0.000 0.000 1.000 2.000 2.000 1.000 1.000 1.000 2.000 3.000 2.000 1.000 0.000 1.000 2.000 2.000

A.zeroDiagonalsInPlace()
B.zeroDiagonalsInPlace()

A = 0.000 0.000 0.000 1.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 0.000 B = 0.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 0.000 2.000 1.000 1.000 1.000 2.000 0.000 2.000 1.000 0.000 1.000 2.000 0.000

A.subtract(B)

0.000 0.000 0.000 0.000 0.000 1.000 0.000 -1.000 1.000 1.000 0.000 -1.000 0.000 -2.000 -1.000 -1.000 -1.000 -2.000 0.000 -2.000 -1.000 0.000 -1.000 -2.000 0.000

[vkostyukov]

Hi @rabbanyk! Thanks for the report. What happens if you subtract B from B w/o calling zeroDiagonalsInPlace(). Does it work?

[rabbanyk]

The result for A-B before setting diagonal elements to zero is correct, i.e. 0.000 0.000 0.000 0.000 0.000 0.000 0.000 -1.000 -1.000 0.000 0.000 -1.000 -1.000 -2.000 -1.000 0.000 -1.000 -2.000 -2.000 -1.000 0.000 0.000 -1.000 -1.000 -1.000

[vkostyukov]

Seemed to be a bug, I will have a look on it today. Did you try the same scenario with CCSMatrix?

[rabbanyk]

No, but I think the problem is with setting an element to zero. Same happens with

public void zeroDiagonalsInPlace() {
    for(int i =0; i<this.rows();i++)
            this.set(i, i, 0);
}

[vkostyukov]

Really interesting. I will try to troubleshot it today. If you find a root-cause - just let me know.

[vkostyukov]

BTW, can you try to call zeroDiagonalsInPlace and then print matrix using iterator? For example:

MatrixIterator it = a.nonZeroIterator();
while (it.hasNext()) {
  double x = it.next();
  int i = it.rowIndex();
  int j = it.columnIndex();
  System.out.println("A[" + i + "," + j + "] = " + x);
}

[rabbanyk]

for A it gives: 0.000 0.000 0.000 1.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 0.000

[0,3] = 1.0 [0,4] = 1.0 [1,0] = 1.0 [1,4] = 1.0 [1,0] = 1.0 [1,3] = 1.0

And for B: 0.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 1.000 0.000 0.000 1.000 0.000 2.000 1.000 1.000 1.000 2.000 0.000 2.000 1.000 0.000 1.000 2.000 0.000

[0,3] = 1.0 [0,4] = 1.0 [1,2] = 1.0 [1,3] = 1.0 [2,1] = 1.0 [2,3] = 2.0 [2,4] = 1.0 [3,0] = 1.0 [3,1] = 1.0 [3,2] = 2.0 [3,4] = 2.0 [4,0] = 1.0 [4,2] = 1.0 [4,3] = 2.0

[vkostyukov]

Which is wrong, right? The issue with set(i, j, 0.0) method. Thanks for the report. I will fix it!

[rabbanyk]

Yes, and Thanks.

[rabbanyk]

I think the problem is with having a row with all zero elements. See for example the following which is not using the set method:

        double [][] a = {{1,0,1,0,0},{0,0,0,0,0},{0,0,0,0,0},{1,0,0,0,0},{1,0,0,1,0}};
        CRSMatrix m = new CRSMatrix(a);
        System.err.println(m);

        MatrixIterator it = m.nonZeroIterator();
        while (it.hasNext()) {
              double x = it.next();
              int i = it.rowIndex();
              int j = it.columnIndex();
              System.err.println("[" + i + "," + j + "] = " + x);
            }
        return;

1.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 1.000 0.000

[0,0] = 1.0 [0,2] = 1.0 [1,0] = 1.0 [1,0] = 1.0 [1,3] = 1.0

[vkostyukov]

Right. I've reproduced it using the similar test. The issue is nonZeroIterator.

[vkostyukov]

Here come the fix! Thanks @rabbanyk! Grab the sources from repo and run mvn install and depend you source code on version 0.5.5. Let me know if you face the issue with javadoc generation.