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 C = A.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.