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

Not sure if this is part of your question, but to construct the appropriate element-wise inequality result you must first cast your matrices to arrays:

MatrixXd A,B;
...
Matrix<bool,Dynamic,Dynamic> C = A.array()<B.array();

Now C is the same size as A and B and C(i,j) = A(i,j) < B(i,j).

To find all of the indices (assuming column-major order) of the true entries, you can use this compact c++11 routine---as described by libigl's conversion table:

VectorXi I = VectorXi::LinSpaced(C.size(),0,C.size()-1);
I.conservativeResize(std::stable_partition(
  I.data(), I.data()+I.size(), [&C](int i){return C(i);})-I.data());

Now I is C.nonZeros() long and contains indices of the true entries in C. These two lines essentially implement find.

Answer 2

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<algorithm>
#include<vector>
#include <Eigen/Dense>
using namespace Eigen;
using namespace std;

class isGreater{
    public:
    vector<int>* GT;
    isGreater(vector<int> *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<int> GT;
    for_each(P.data(),P.data()+P.rows()*P.cols(),isGreater(&GT));
    cout<<P<<endl;
    for(int i=0;i<GT.size();++i)cout<<GT[i]<<" ";
    cout<<GT.size()<<endl;
    return 0;
}

Answer 3

This might work for you and others who check this out. In order to set elements of a matrix m based on the condition on another matrix A, you can use this notation:

m = (A.array() != 0).select(1, m);

This command replaces those elements in matrix m that have non-zero corresponding elements in A, with one.