dft_tools/doc/reference/clef/assign_0.cpp

#include "triqs/clef.hpp"
#include <iostream>
using namespace triqs::clef;

int main() {
 int N = 5;
 double pi = std::acos(-1);

 // automatic assignment of vector
 placeholder<0> k_;
 std::vector<double> V(N);
 make_expr(V)[k_] << cos((2 * pi * k_) / N);

 // chaining them ...
 placeholder<1> i_;
 std::vector<std::vector<double>> W(3, std::vector<double>(N));
 make_expr(W)[i_][k_] << i_ + cos((2 * pi * k_) / N);

 // check result...
 for (size_t u = 0; u < V.size(); ++u)
  if (std::abs(V[u] - cos((2 * pi * u) / N)) > 1.e-10) throw "error!";
 for (size_t w = 0; w < W.size(); ++w)
  for (size_t u = 0; u < W[w].size(); ++u)
   if (std::abs(W[w][u] - (w + cos((2 * pi * u) / N))) > 1.e-10) throw "error!";
}