Eigen MatrixX assertion failed though resize is called - c++

Question

I have some problems with the Eigen library. When I try t access the matrices in the second for loop, I get the
eigen assertion index = 0 && index size() failed
error though I am calling the resize function of the Matrix.
Eigen::Matrix<float, 1, 2> linearRegression(Eigen::Vector2f *p, int pointCount)
{
Eigen::MatrixXf M;
Eigen::Matrix<float, 1, 2> A;
Eigen::MatrixXf Y;
M.resize(pointCount, 2);
Y.resize(pointCount, 1);
for (int i = 0; i < pointCount; i++)
{
M(i, 0) = p[i].x();
M(i, 1) = 1;
Y(i, 0) = p[i].y();
}
A = (M.transpose() * M).inverse() * M.transpose() * Y;
return A;
}
I can see the members m_rows and m_cols of matrix M in the visual studio debugger and i does not exceed them. The error occurs directly at i = 0.
MatrixXf should be the same as Matrix<float, Dynamic, Dynamic>.
When I declare a constant pointCount and use fixed size matrices, everything works fine.
Eigen::Matrix<float, 1, 2> linearRegression(Eigen::Vector2f *p, int pointCount)
{
const int pointCount = 30;
...
Eigen::Matrix<float, pointCount, 2> M;
Eigen::Matrix<float, 1, 2> A;
Eigen::Matrix<float, pointCount, 1> Y;
...
}
What am I doing wrong?

Answer 1

Your expression (M.transpose() * M).inverse() * M.transpose() * Y results in a matrix with 2 rows and 1 column, while A has 1 row and 2 columns. Because A is a fixed size, the resizing doesn't happen upon assignment. To fix, add a transpose to your expression:
((M.transpose() * M).inverse() * M.transpose() * Y).transpose()