Question

I have a function in a generic class that calculates a determinant of a matrix. The function works for some inputs with any type, and for others it gives the wrong answer (based on the type).

Here is the function:

    public T Determinant()
    {
        checked
        {
            int n = dimension;
            Matrix<T> a = new Matrix<T>(baseArray);
            int i, j, k;
            T det = (dynamic)0;
            for (i = 0; i < n - 1; i++)
            {
                for (j = i + 1; j < n; j++)
                {
                    det = (dynamic)a[j, i] / a[i, i];
                    for (k = i; k < n; k++)
                        a[j, k] = a[j, k] - (dynamic)det * a[i, k];
                }
            }
            det = (dynamic)1;
            for (i = 0; i < n; i++)
                det = (dynamic)det * a[i, i];

            return det;
        }
    }

I added the checked block to see if there was an overflow taking place, but apparently, there is no overflow happening.

A sample input is if the baseArray is new double[,] {{11, 11, 12, 17, 21, 29}, {16, 9, 25, 30, 29, 33}, {3, 13, 9, 24, 21,
24}, {23, 6, 29, 21, 23, 23}, {22, 19, 14, 30, 21, 24}, {22, 28,
20, 17, 25, 28}}; it will give the right answer (very close to -942755), but if baseArray is instead new int[,] {{11, 11, 12, 17, 21, 29}, {16, 9, 25, 30, 29, 33}, {3, 13, 9, 24, 21,
24}, {23, 6, 29, 21, 23, 23}, {22, 19, 14, 30, 21, 24}, {22, 28,
20, 17, 25, 28}}; it gives 15934050 as the answer (not even close).

The indexer on the matrix just returns the i-th j-th elements of the matrix, so they are not the issue.

I’m baffled as to what the issue could be since it is not an overflow. Any ideas?

Code to reproduce:

public class Matrix<T>
where T : IConvertible
{
    private int dimension;
    private T[][] baseArray;

    public Matrix(int dimensions, T[,] baseArray)
    {
        this.dimension = dimensions;
        this.baseArray = new T[dimension][];
        for (int i = 0; i < dimension; i++)
        {
            this.baseArray[i] = new T[dimension];
            for (int j = 0; j < dimension; j++)
            {
                this[i, j] = baseArray[i, j];
            }
        }
    }

    public T this[int a, int b]
    {
        get
        {
            return baseArray[a][b];
        }
        set
        {
            baseArray[a][b] = value;
        }
    }

    public T Determinant()
    {
        checked
        {
            int n = dimension;
            Matrix<T> a = new Matrix<T>(baseArray);
            int i, j, k;
            T det = (dynamic)0;
            for (i = 0; i < n - 1; i++)
            {
                for (j = i + 1; j < n; j++)
                {
                    det = (dynamic)a[j, i] / a[i, i];
                    for (k = i; k < n; k++)
                        a[j, k] = a[j, k] - (dynamic)det * a[i, k];
                }
            }
            det = (dynamic)1;
            for (i = 0; i < n; i++)
                det = (dynamic)det * a[i, i];

            return det;
        }
    }
}

Answer 1

When the values are int values, it performs integer arithmetic.
When the values are double values, it performs floating point arithmetic – it’s as simple as that. Simpler example:

using System;

public class Test
{
    static void Main()
    {
        PrintResult(1, 2);      // Prints 0
        PrintResult(1.0, 2.0);  // Prints 0.5
    }

    static void PrintResult(dynamic x, dynamic y)
    {
        Console.WriteLine(x / y);
    }
}