MATLAB find() / Numpy nonzero idioms for Eigen

Question

Chances are this is a very stupid question but I spent a pretty absurd amount of time looking for it on the documentation, to no avail.

in MATLAB, the find() function gi

Answer 1

It is reasonable to expect Eigen to have a find() function. Unfortunately, Eigen doesn't have one, or even a less than operator for matrices. Fortunately, the problem isn't too difficult. Here is one solution to the problem. I am using vector to store the Column Major indices of elements > 0. You could use VectorXf if you prefer that. Use this on B - A (B-A > 0 is the same as evaluating B>A). I'm using the stl for_each() function.

#include
#include
#include 
using namespace Eigen;
using namespace std;

class isGreater{
    public:
    vector* GT;
    isGreater(vector *g){GT = g;}
    void operator()(float i){static int it = 0; if(i>0)GT->push_back(it); it++;}
};
int main(int argc,char **argv){
    MatrixXf P = MatrixXf::Random(4,5);
    vector GT;
    for_each(P.data(),P.data()+P.rows()*P.cols(),isGreater(>));
    cout<