Eigen how to concatenate matrix along a specific dimension?

Question

I have two eigen matrices and I would like to concatenate them, like in matlab cat(0, A, B)

Is there anything equivalent in eigen?

Thanks.

Answer 1

I had a slightly different use case: To vertically stack a std::vector of Eigen matrices. Here is how I implemented a function which is more general purpose. Let me know if this can be further improved:

// matrix_eig = Eigen::MatrixXf in RowMajor format
matrix_eig VStack(const std::vector &mat_vec) {
  assert(!mat_vec.empty());
  long num_cols = mat_vec[0].cols();
  size_t num_rows = 0;
  for (size_t mat_idx = 0; mat_idx < mat_vec.size(); ++mat_idx) {
    assert(mat_vec[mat_idx].cols() == num_cols);
    num_rows += mat_vec[mat_idx].rows();
  }
  matrix_eig vstacked_mat(num_rows, num_cols);
  size_t row_offset = 0;
  for (size_t mat_idx = 0; mat_idx < mat_vec.size(); ++mat_idx) {
    long cur_rows = mat_vec[mat_idx].rows();
    vstacked_mat.middleRows(row_offset, cur_rows) = mat_vec[mat_idx];
    row_offset +=  cur_rows;
  }
  return vstacked_mat;
}