eigen resize

Eigen is a C++ template library for linear algebra operations. The `resize` function in Eigen is used to change the size of a matrix or vector.

To resize a matrix or vector in Eigen, you can use the `resize` member function. Here's an example:

#include <iostream>
#include <Eigen/Dense>

int main() {
  Eigen::MatrixXd matrix(3, 3);
  std::cout << "Original size: " << matrix.rows() << "x" << matrix.cols() << std::endl;

  matrix.resize(4, 4);
  std::cout << "Resized size: " << matrix.rows() << "x" << matrix.cols() << std::endl;

  return 0;
}

In the above example, we initially create a 3x3 matrix using `Eigen::MatrixXd`. We then resize it to a 4x4 matrix using the `resize` function. The `rows()` and `cols()` member functions are used to retrieve the dimensions of the matrix.

Note that resizing a matrix can result in the loss of data if the new size is smaller than the original size.