Eigen: how can I substitute matrix positive values with 1 and 0 otherwise? - matlab

Question

I want to write the following matlab code in Eigen (where K is pxp and W is pxb):
H = (K*W)>0;
However the only thing that I came up so far is:
H = ((K*W.array() > 0).select(1,0));
This code doesn't work as explained here, but replacing 0 with VectorXd::Constant(p,0) (as suggested in the link question) generates a runtime error:
Eigen::internal::variable_if_dynamic<T, Value>::variable_if_dynamic(T) [with T = long int; int Value = 1]: Assertion `v == T(Value)' failed.
How can I solve this?

Answer 1

You don't need .select(). You just need to cast an array of bool to an array of H's component type.
H = ((K * W).array() > 0.0).cast<double>();
Your original attempt failed because the size of your constant 1/0 array is not match with the size of H. Using VectorXd::Constant is not a good choice when H is MatrixXd. You also have a problem with parentheses. I think you want * rather than .* in matlab notation.
#include <iostream>
#include <Eigen/Eigen>
using namespace Eigen;
int main() {
const int p = 5;
const int b = 10;
MatrixXd H(p, b), K(p, p), W(p, b);
K.setRandom();
W.setRandom();
H = ((K * W).array() > 0.0).cast<double>();
std::cout << H << std::endl << std::endl;
H = ((K * W).array() > 0).select(MatrixXd::Constant(p, b, 1),
MatrixXd::Constant(p, b, 0));
std::cout << H << std::endl;
return 0;
}
When calling a template member function in a template, you need to use the template keyword.
#include <iostream>
#include <Eigen/Eigen>
using namespace Eigen;
template<typename Mat, typename Vec>
void createHashTable(const Mat &K, Eigen::MatrixXi &H, Mat &W, int b) {
Mat CK = K;
H = ((CK * W).array() > 0.0).template cast<int>();
}
int main() {
const int p = 5;
const int b = 10;
Eigen::MatrixXi H(p, b);
Eigen::MatrixXf W(p, b), K(p, p);
K.setRandom();
W.setRandom();
createHashTable<Eigen::MatrixXf, Eigen::VectorXf>(K, H, W, b);
std::cout << H << std::endl;
return 0;
}
See this for some explanation.
Issue casting C++ Eigen::Matrix types via templates