I keep getting that assertion failure when I run my code. Can anyone tell me whats wrong here? The code is a simple forward substitution function:

Code: Select all

Eigen::VectorXd forward_subst(Eigen::MatrixXd L, Eigen::VectorXd b)
{
   Eigen::VectorXd x;

   x(0) = b(0)/L(0, 0);

   for(int k = 1; k < L.cols(); k++)
   {
      double sum = 0;
      for(int i = 0; i < k - 1; i++)
         sum = sum + L(k, i)*x(i);
      x(k) = (b(k) - sum)/L(k, k);
   }

   return x;
}

And the error at runtime is:
rich@rich-VirtualBox:~/workspace/linalgtest/Debug$ ./linalgtest
linalgtest: /usr/include/eigen3/Eigen/src/Core/DenseCoeffsBase.h:407: Eigen::DenseCoeffsBase<Derived, 1>::Scalar& Eigen::DenseCoeffsBase<Derived, 1>::operator()(Eigen::DenseCoeffsBase<Derived, 1>::Index) [with Derived = Eigen::Matrix<double, -0x00000000000000001, 1>, Eigen::DenseCoeffsBase<Derived, 1>::Scalar = double, Eigen::DenseCoeffsBase<Derived, 1>::Index = long int]: Assertion `index >= 0 && index < size()' failed.
Aborted (core dumped)

You need to specify the size of the vector x. If you declare x as you did, it will be an empty vector (a vector of length 0), so you get an error when you try to access its first element.