How to use cwise operations over specific indexes of a vector? (Eigen)

Question

I'm trying to translate the following Matlab code to C/C++.

indl = find(dlamu1 < 0);  indu = find(dlamu2 < 0);
s = min([1; -lamu1(indl)./dlamu1(indl); -lamu2(indu)./dlamu2(indu)]);

I've read on another thread that there's yet no equivalent in the Eigen library to the find() function and I'm at peace with that and have brute-forced around it.

Now, if I wanted to do the coefficient-wise division of lamu1 and dlamu1, I'd go for lamu1.cwiseQuotient(dlamu1) but how do I go about doing that but only for some of their coefficients, which indexes are specified by the coefficients of indl? I haven't found anything about this in the documentation, but maybe I'm not using the right search terms.

Answer 1

With the default branch you can just write lamu1(indl) with indl a std::vector<int> or a Eigen::VectorXi or whatever you like that supports random access through operator[].