Lambda Function Elementwise in Eigen

Question

This is a combination of the following previous questions: Apply function to all Eigen matrix element and Set coefficients of an Eigen::Matrix according an arbitrary distribution. Basically I'm trying to generate an Eigen matrix with its coefficients sampled from a Gaussian distribution.

Here is my code to do this(static class method) which returns a rather cryptic error message:

matrix_eig EigenUtil::GaussianNoise(size_t rows, size_t cols,
                                    float mean, float std) {
  matrix_eig m(rows, cols);
  std::mt19937 rng;
  std::normal_distribution<float> nd(mean, std);
  auto sampler = [&]() { return nd(rng); };
  return matrix_eig::Zero(rows, cols).unaryExpr(sampler);
}

Which returns the the error: error:

no type named 'type' in 'std::__1::result_of<(lambda at eigen_util.cpp:101:18) (const float &)>'
  typedef typename std::result_of<T>::type type1;

Answer 1

As o11c noticed, this is indeed a nullary-expression, and there is almost the exact same example in the doc. I copied it for convenience:

#include <Eigen/Core>
#include <iostream>
#include <random>
using namespace Eigen;
int main() {
  std::default_random_engine generator;
  std::poisson_distribution<int> distribution(4.1);
  auto poisson = [&] () {return distribution(generator);};
  RowVectorXi v = RowVectorXi::NullaryExpr(10, poisson );
  std::cout << v << "\n";
}