I'm new to c++ and Eigen. I would like to know if I can use NullaryExpr to fill a matrix with the results of a function that changes state everytime the () operator is used.

Thank you for your help.

yes, this is how random is implemented.

Thank you for the quick response. The problem is that when I try to compile my function I get the following error:

c:\program files (x86)\eigen-eigen-3.0.2\eigen\src/Core/CwiseNullaryOp.h (95): error C3848: expression having type 'const fill<Scalar>' would lose some const-volatile qualifiers in order to call 'double fill<Scalar>::operator ()(Eigen::DenseIndex &)'

Here is my code:

Code: Select all

template<typename Scalar>
class fill
{
public:
   fill(Scalar n_)
      : counter(n_)
   {}

   EIGEN_EMPTY_STRUCT_CTOR(fill)
   inline Scalar operator() (Eigen::DenseIndex& a) { return ++n; };  //generaters error
   //inline Scalar operator() (Eigen::DenseIndex& a) const { return 1; }; compiles OK
   
protected:
   Scalar counter;
};

namespace Eigen
{
namespace internal
{
   template<typename Scalar>
   struct functor_traits<fill<Scalar> >
   {
      enum { Cost = 1, PacketAccess = false, IsRepeatable = false};
   };
}
}

int main()
{
   Eigen::Matrix<double,5,5> x = Eigen::Matrix<double,5,5>::NullaryExpr( fill<double>(1.0) );
    return 0;
}

thank you again for your help. I suspect this is a silly question but I cannot figure it out.

I think that operator() has to be const qualified, and thus counter should be mutable.

btw, toy can use setLinSpace(...) to reproduce what you want to achieve.

Thank you. I'm now using mutable and everything is working fine.