joaocandre - 10 months ago 70

C++ Question

I am using MatrixXd matrices from Eigen on my code, and at a certain point I need a 3D one. Since Eigen does not have tridimensional matrix types, as it is optimized just for linear algebra, instead I am creating a pointer array of the MatrixXd type:

`Eigen::MatrixXd *CVM =new Eigen::MatrixXd[100];`

for (int i = 0; i < 100; i++){

CVM[i]= Eigen::MatrixXd::Zero(5,5);

}

However, later on I need to acess the values on this array, and for that I am doing something like:

`for (int k = 0; k < 100; k++){`

Eigen::MatrixXd* b=&CVM[k];

for (int i = 0; i < 5; i++){

for (int j = 0; j < 5; j++){

b->coeff(i,j)=47;

}

}

}

As

`b`

`MatrixXd`

`b(i,j)`

`coeff()`

`error: assignment of read-only location ‘b->Eigen::Matrix<double, -1, -1>::<anonymous>.Eigen::PlainObjectBase<Derived>::coeff<Eigen::Matrix<double, -1, -1> >(((Eigen::PlainObjectBase<Eigen::Matrix<double, -1, -1> >::Index)i), ((Eigen::PlainObjectBase<Eigen::Matrix<double, -1, -1> >::Index)j))’`

EDIT: output added

`cout << b << endl;`

cout << CVM[47] << endl;

0x1c34b00

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

Answer Source

Either use `b->coeffRef(i,j)`

to get a read/write reference, or dereference b: `(*b)(i,j)`

, or use a reference for b:

```
MatrixXd& b = CVM[k];
b(i,j) = 47;
```