CODEWITHSUNDEEP
c++memoryeigen
Julien
Julien

Reputation: 15071

How to pre-allocate memory for a growing Eigen::MatrixXd

I have a growing database in form of an Eigen::MatrixXd. My matrix starts empty and gets rows added one by one until it reaches a maximum predefined (known at compile time) number of rows.

At the moment I grow it like that (from the Eigen docs and many posts here and elsewhere):

MatrixXd new_database(database.rows()+1, database.cols());
new_database << database, new_row;
database = new_database;

But this seems way more inefficient than it needs to be, since it makes a lot of useless memory reallocation and data copying every time a new row is added... Seems like I should be able to pre-allocate a bunch of memory of size MAX_ROWS*N_COLS and let the matrix grow in it, however I can't find an equivalent of std::vector's capacity with Eigen.

Note: I may need to use the matrix at anytime before it is actually full. So I do need a distinction between what would be its size and what would be its capacity.

How can I do this?

EDIT 1: I see there is a MaxSizeAtCompileTime but I find the doc rather unclear with no examples. Anyone knows if this is the way to go, how to use this parameter and how it would interact with the resize and conservativeResize?

EDIT 2: C++: Eigen conservativeResize too expensive? provides another interesting approach while raising question regarding non contiguous data... Anyone has some good insight on that matter?

Upvotes: 2

Views: 2688

Answers (1)

Avi Ginsburg
Avi Ginsburg

Reputation: 10596

First thing I want to mention before I forget, is that you may want to consider using a row major matrix for storage.

The simplest (and probably best) solution to your question would be to use block operations to access the top rows. 

#include <Eigen/Core>
#include <iostream>

using namespace Eigen;

int main(void)
{
    const int rows = 5;
    const int cols = 6;

    MatrixXd database(rows, cols);
    database.setConstant(-1.0);

    std::cout << database << "\n\n";

    for (int i = 0; i < rows; i++)
    {
        database.row(i) = VectorXd::Constant(cols, i);

        // Use block operations instead of the full matrix
        std::cout << database.topRows(i+1) << "\n\n";

    }

    std::cout << database << "\n\n";

    return 0;
}

Instead of just printing the matrix, you could do whatever operations you require.

Upvotes: 2

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