Calculating the Determinant in C++ - c

Question

I was trying to calculate the determinant of a 3 * 3 matrix (or more) with the matrix values ranging from (-1, to 1). However, I get a result of 0 when I calculate the determinant.
[...]
srand(time(NULL));
//Random generation of values between -1 and 1
for(i = 0; i < 3; i++)
{
for(j = 0; j < 3; j++)
{
temp = (rand() % (500)) + 0;
temp = temp/250;
array[i][j] = (temp - 1);
}
[...]
double array2[10][10];
double detrm = 0;
int s = 1;
int i, j, m, n, c;
for (c = 0; c < x; c++)
{
m = 0;
n = 0;
for (i = 0; i < x; i++)
{
for (j = 0; j < x; j++)
{
array2[i][j] = 0;
if (i != 0 && j != c)
{
array2[m][n] = a[i][j];
if ( n < (x - 2))
{
n++;
}
else
{
n = 0;
m++;
}
}
}
}
detrm = detrm + (s*a[0][c]*determinant(array2, (x - 1)));
s = -1*s;
}
return(detrm);

Answer 1

This may Help - see comments within the code for an explanation:
static int CalcDeterminant(vector<vector<int>> Matrix)
{
//this function is written in c++ to calculate the determinant of matrix
// it's a recursive function that can handle matrix of any dimension
int det = 0; // the determinant value will be stored here
if (Matrix.size() == 1)
{
return Matrix[0][0]; // no calculation needed
}
else if (Matrix.size() == 2)
{
//in this case we calculate the determinant of a 2-dimensional matrix in a
//default procedure
det = (Matrix[0][0] * Matrix[1][1] - Matrix[0][1] * Matrix[1][0]);
return det;
}
else
{
//in this case we calculate the determinant of a squared matrix that have
// for example 3x3 order greater than 2
for (int p = 0; p < Matrix[0].size(); p++)
{
//this loop iterate on each elements of the first row in the matrix.
//at each element we cancel the row and column it exist in
//and form a matrix from the rest of the elements in the matrix
vector<vector<int>> TempMatrix; // to hold the shaped matrix;
for (int i = 1; i < Matrix.size(); i++)
{
// iteration will start from row one cancelling the first row values
vector<int> TempRow;
for (int j = 0; j < Matrix[i].size(); j++)
{
// iteration will pass all cells of the i row excluding the j
//value that match p column
if (j != p)
{
TempRow.push_back(Matrix[i][j]);//add current cell to TempRow
}
}
if (TempRow.size() > 0)
TempMatrix.push_back(TempRow);
//after adding each row of the new matrix to the vector tempx
//we add it to the vector temp which is the vector where the new
//matrix will be formed
}
det = det + Matrix[0][p] * pow(-1, p) * CalcDeterminant(TempMatrix);
//then we calculate the value of determinant by using a recursive way
//where we re-call the function by passing to it the new formed matrix
//we keep doing this until we get our determinant
}
return det;
}
}
};

Answer 2

You have a crufty way of updating m and n. You should increment m in the outer loop over i, and initialize n within the outer loop and increment it within the inner loop. I think that your code would work as you wrote it, but I think that your conditional should have been i < n-1 instead of i < n-2. But instead of changing the fewest number of characters to get the code to work, I recommend restructuring the increments so that the issue doesn't arise.