# Convert Eigen Matrix to C array

You can use the data() member function of the Eigen Matrix class. The layout by default is column-major, not row-major as a multidimensional C array (the layout can be chosen when creating a Matrix object). For sparse matrices the preceding sentence obviously doesn't apply.

Example:

```
ArrayXf v = ArrayXf::LinSpaced(11, 0.f, 10.f);
// vc is the corresponding C array. Here's how you can use it yourself:
float *vc = v.data();
cout << vc[3] << endl; // 3.0
// Or you can give it to some C api call that takes a C array:
some_c_api_call(vc, v.size());
// Be careful not to use this pointer after v goes out of scope! If
// you still need the data after this point, you must copy vc. This can
// be done using in the usual C manner, or with Eigen's Map<> class.
```

To convert normal data type to eigen matrix type

```
double *X; // non-NULL pointer to some data
```

You can create an nRows x nCols size double matrix using the Map functionality like this:

```
MatrixXd eigenX = Map<MatrixXd>( X, nRows, nCols );
```

To convert eigen matrix type into normal data type

```
MatrixXd resultEigen; // Eigen matrix with some result (non NULL!)
double *resultC; // NULL pointer <-- WRONG INFO from the site. resultC must be preallocated!
Map<MatrixXd>( resultC, resultEigen.rows(), resultEigen.cols() ) = resultEigen;
```

In this way you can get in and out from eigen matrix. Full credits goes to http://dovgalecs.com/blog/eigen-how-to-get-in-and-out-data-from-eigen-matrix/