CODEWITHSUNDEEP
c++eigen
Jared Elinger
Jared Elinger

Reputation: 33

How to Define an Eigen Matrix size based upon a variable passed to a function

I am trying to write code that needs to define a matrix size based upon an input. A stripped down version of the code that shows the issue is:

 #include <iostream>
    #include "eigen/Eigen/Dense"
    #include <cmath>

    using namespace Eigen;

    void matrixname(  const int numbRow, const int numbcol);

    int main()
    {
    const int numbRow=5;
    const int numbCol=3;

    matrixname(numbRow,numbCol);
    return 0;
    }

    void matrixname(  const int numbRow, const int  numbCol)
    {
    Matrix<double,numbRow,numbCol> y;
    }

When attempting to compile the code, the following error is returned:

/main.cpp:20:15: error: non-type template argument is not a constant expression

The build breaks in the last line of trying to define y.

Is there any way I can modify the declaration or passing of the variables in order to be able to define the size of the matrix in this way?

Upvotes: 3

Views: 1637

Answers (1)

user6764549
user6764549

Reputation:

As per the documentation, if you don't know the size of a matrix at compile time you need to use the matrix size template parameters as Eigen::Dynamic.

So you may have to modify your function as follows:

void matrixname(  const int numbRow, const int  numbCol)
{
    Matrix<double,Eigen::Dynamic,Eigen::Dynamic> y1(numbRow, numbCol);

    // Eigen also provides a typedef for this type
    MatrixXd y2(numbRow, numbCol);
}

Upvotes: 1

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