Find difference between two lists using Eigen - c++

Question

I would like to find the difference between two lists. For example:
// two lists:
A = [ 0, 1, 2, 3, 4, 5, 6 ];
B = [ 1, 4, 5 ];
// difference between the lists:
C = [ 0, 2, 3, 6 ];
I have done this using the STL-library of C++ as follows:
#include <iostream>
#include <vector>
int main()
{
std::vector<size_t> A = {0, 1, 2, 3, 4, 5, 6};
std::vector<size_t> B = {1, 4, 5};
std::vector<size_t> C;
std::set_difference(A.begin(),A.end(), B.begin(),B.end(), std::inserter(C,C.begin()));
return 0;
}
However, because my application uses mostly Eigen, I now would like to do also this using Eigen. I couldn't find what I was looking for in the documentation nor online.
Note that I specifically want to avoid writing my own function.

Answer 1

Here you go:
#include <iostream>
#include <Eigen/Dense>
int main()
{
using namespace Eigen;
VectorXd a(3), b(1);
VectorXd c(a.size());
a << 1,2,3;
b << 1;
auto it = std::set_difference(a.data(), a.data() + a.size(),
b.data(), b.data() + b.size(),
c.data());
c.conservativeResize(std::distance(c.data(), it)); // resize the result
std::cout << c;
}
The key here is to use Eigen::VectorXd::data() member function, which returns a pointer to the underlying storage, which is itself an iterator that can be passed around to C++ standard library functions.