Using Eigen vectors in Matlab mex files

Question

I am trying to use the Eigen library for a c++ that I want to make into a mex file in Matlab. I broke the code down to its essentials below. I need to make a return vector of which the length corresponds to the number of rows from an input matrix.

With the code below I get the follwing errors on compiling corresponding to the line:

double y_OUT[nrow] = {};

• error C2057: expected constant expression-error
• error C2466: cannot allocate an array of constant size 0

I can't figure out why I cannot allocate the vector of length nrow. By uncommenting the specific line and by printing nrow i verified that it actually contains the right number. Does anyone can give any pointers (pun intended)?

     void mexFunction(
             int          nlhs,
             mxArray      *plhs[],
             int          nrhs,
             const mxArray *prhs[]
             )
    {



      double *x_IN;
      int nrow,ncols;

      /* Check for proper number of arguments */
      //...

      x_IN = mxGetPr(prhs[0]);
      nrow = (int)mxGetM(prhs[0]);
      ncols = (int)mxGetN(prhs[0]);

      double y_OUT[nrow] = {};

      MatrixXd x=Map<MatrixXd>(x_IN,nrow,ncols);
      VectorXd Respons=VectorXd::Zero(nrow);

      Map<VectorXd>(y_OUT,nrow)=Respons.array();
      return;
    }

Answer 1

You are not allocating nothing in there. The line double y_OUT[nrow] = {}; contains two errors. As described by the error messages.

1. nrow is not constexpr. It cannot be evaluated at compile time.
2. by the braces initializer you suggest the vector has 0 elements, it does not make sense either.

The proper syntax for dynamic allocation is: double* y_OUT = new double[nrow];. I think that is what you wanted. The syntax you used double y_OUT[nrow] is for used for static allocation, in other words where the size of array must be known at compile time.

Sidenotes: Of course you need to remember to free the memory you reserve. And the problem is totally unrelated to the topic IMO =)