Append column to matrix, using Eigen library

Question

It's quite a simple task, but I was not able to find an answer to it:

Using the Eigen library, suppose I have Matrix2Xd mat and Vector2d vec, where

mat = 1 1 1
      1 1 1
vec = 2 2

Now I need something like mat.addCol(vec) such that afterwards

mat = 1 1 1 2
      1 1 1 2

What is the best (simplest) way to accomplish this?

Please note, that this is not a duplicate of How do you make a matrix out of vectors in eigen?. I don't want to initialy construct the matrix but append to an existing one. Or is there maybe a trick, how to use the comma initialization in this case? The following code will fail:

Matrix2Xd mat(2,3);
Vector2d vec;
mat << 1, 1, 1, 1, 1, 1;
vec << 2, 2;

cout << mat << endl;
mat << vec;             // <-- crashes here
cout << mat << endl;

---

Edit: The following works, but I don't like the need of a temporary variable for such a basic task. Is there a better way?

Matrix2Xd tmp(2, mat.cols()+1);
tmp << mat, vec;
mat = tmp;

Burtburta

You did read the first answer in the dupe: "The matrix m mus have been properly resized first."? An Eigen Matrix has a resize member function... Documentation can be googled, or read, at the least. – Cherie

@Cherie resize does not keep the content of the matrix. If I do mat.resize(2, mat.cols()+1);, the matrix is filled with some random values afterwards. – Burtburta

Then you'll need to either create a temporary or initialize it to the correct size. – Cherie

@Cherie The size is not known in the beginning, so this is not possible. I hoped there is a better solution than copying all the data to a temporary :/ – Burtburta

There is no data structure (that is contiguous in memory, as would be wanted for a numerical matrix) that fullfills your needs. What sizes are we talking about? – Cherie

@Cherie Well, for example the matrix class of the Armadillo library has insert_cols() and insert_rows(), so I hoped there is something similar in Eigen. But I'm happy with ggaels answer, so I think we can end this discussion. – Burtburta

just so you know, those functions might well reallocate the memory, you're just "hiding" that fact from plain view. See e.g. this question for some more info. – Cherie

Answer 1

You can use conservativeResize for that purpose:

mat.conservativeResize(mat.rows(), mat.cols()+1);
mat.col(mat.cols()-1) = vec;

Lath

Thanks, this is working. I figured out that it is even possible to write mat.conservativeResize(NoChange, mat.cols()+1); which lets the number of rows unchanged. – Burtburta

Is this approach recommended from a performance perspective? As I understand, conservativeResize() will allocate memory again and copy old data to the new location. Suppose we need to append multiple vectors to the matrix, does this method suffer from performance loss significantly? – Humphries