[jira] [Created] (MATH-1617) FieldLUDecomposition with BigReal throw divide by zero error

[Ng Tsz Sum (Jira)]

Ng Tsz Sum created MATH-1617:
--------------------------------

             Summary: FieldLUDecomposition with BigReal throw divide by zero 
error
                 Key: MATH-1617
                 URL: https://issues.apache.org/jira/browse/MATH-1617
             Project: Commons Math
          Issue Type: Bug
    Affects Versions: 3.6.1
            Reporter: Ng Tsz Sum


>From 
>[https://stackoverflow.com/questions/68427448/how-to-find-the-inverse-of-a-matrix-using-apache-commons-math-library-in-java/68427869?noredirect=1#comment120932354_68427869]

h3. Reproducible example

{code:java}
import org.apache.commons.math3.linear.FieldLUDecomposition;
import org.apache.commons.math3.linear.FieldMatrix;
import org.apache.commons.math3.linear.MatrixUtils;
import org.apache.commons.math3.util.BigReal;

public class REPREX {
    public static void main(String[] args) {
        BigReal[][] leftMatrixData = new BigReal[][]{
                {new BigReal(1), new BigReal(0), new BigReal(0), new 
BigReal(0)},
                {new BigReal(1), new BigReal(0), new BigReal(1), new 
BigReal(0)},
                {new BigReal(1), new BigReal(1), new BigReal(0), new 
BigReal(0)},
                {new BigReal(1), new BigReal(1), new BigReal(1), new 
BigReal(1)},
        };

        FieldMatrix<BigReal> leftMatrix = 
MatrixUtils.createFieldMatrix(leftMatrixData);
        FieldMatrix<BigReal> leftMatrixInverse = new 
FieldLUDecomposition<>(leftMatrix)
                .getSolver()
                .getInverse();
//  Exception in thread "main" 
org.apache.commons.math3.exception.MathArithmeticException: zero not allowed 
here
//        at org.apache.commons.math3.util.BigReal.divide(BigReal.java:255)
//        at org.apache.commons.math3.util.BigReal.divide(BigReal.java:39)
//        at 
org.apache.commons.math3.linear.FieldLUDecomposition.<init>(FieldLUDecomposition.java:160)
//        at stackoverflow.math.matrix.REPREX.main(REPREX.java:18)
    }
}
{code}

h3. Possible reason:

In {{FieldLUDecomposition}} line 130-133
{code:java}
                if (lu[nonZero][col].equals(field.getZero())) {
                    // try to select a better permutation choice
                    ++nonZero;
                }
{code}
Which produce incorrect result when {{lu[nonZeror][col]}} the BigDecimal val 
has different scale 
to {{field.getZero()}} scale. as the {{BigReal#equals}} is comparing using 
{{BigDecimal#equals}}

h3. Workaround

I tried to copy class {{BigReal}} and {{BigRealField}} to {{FixBigReal}} and 
{{FixBigRealField}} and replace all {{BigReal}} to {{FixBigReal}} inside. Then 
override {{FixBIgReal#equals}} as 
{code:java}
    @Override
    public boolean equals(Object other) {
        if (this == other) {
            return true;
        }

        if (other instanceof FixBigReal) {
            return d.compareTo(((FixBigReal) other).d) == 0;
        }
        return false;
    }
{code}
Then the below program will not throw error
{code:java}
import org.apache.commons.math3.linear.FieldLUDecomposition;
import org.apache.commons.math3.linear.FieldMatrix;
import org.apache.commons.math3.linear.MatrixUtils;
import org.apache.commons.math3.util.BigReal;

public class MVE {
    public static void main(String[] args) {
        FixBigReal[][] leftMatrixData = new FixBigReal[][]{
                {new FixBigReal(1), new FixBigReal(0), new FixBigReal(0), new 
FixBigReal(0)},
                {new FixBigReal(1), new FixBigReal(0), new FixBigReal(1), new 
FixBigReal(0)},
                {new FixBigReal(1), new FixBigReal(1), new FixBigReal(0), new 
FixBigReal(0)},
                {new FixBigReal(1), new FixBigReal(1), new FixBigReal(1), new 
FixBigReal(1)},
        };

        FieldMatrix<FixBigReal> leftMatrix = 
MatrixUtils.createFieldMatrix(leftMatrixData);
        FieldMatrix<FixBigReal> leftMatrixInverse = new 
FieldLUDecomposition<>(leftMatrix)
                .getSolver()
                .getInverse();
    }
}
{code}



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