mirror of
https://github.com/triqs/dft_tools
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64 lines
2.4 KiB
C++
64 lines
2.4 KiB
C++
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#include <triqs/clef.hpp>
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#include <triqs/arrays.hpp>
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#include <iostream>
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#include <algorithm>
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int main() {
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// Declaring some placeholders (i.e. dummy variables).
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triqs::clef::placeholder<0> i_;
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triqs::clef::placeholder<1> j_;
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// Declaring a 3x3 matrix
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triqs::arrays::matrix<double> A(3, 3);
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// Automatically filling the matrix
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// -> forget about the bounds, it is automatic
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// -> forget about the best order to order the for loops for performance, it is also automatic
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A(i_, j_) << i_ + 2 * j_;
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// Cheking the result
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std::cout << A << std::endl;
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// It also works for std container: we just have to add a call clef::make_expr function
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std::vector<double> V(10);
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double pi = std::acos(-1);
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// Automatically filling the vector with the evaluation of the expression in i_
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triqs::clef::make_expr(V)[i_] << cos(2 * pi / 5.0 * i_);
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// -> by the way, the constant calculation is precomputed
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// (expressions are partially evaluated as soon as possible)
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// illustration :
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// the time_consuming_function will be called only once in the loop, while cos is called 10 times
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auto time_consuming_function = [](double x) {
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std::cout << "call time_consuming_function" << std::endl;
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return 2 * x;
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};
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triqs::clef::make_expr(V)[i_] << cos(time_consuming_function(10) * i_);
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// If you insist using on more complex containers...
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std::vector<std::vector<double>> W(3, std::vector<double>(5));
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triqs::clef::make_expr(W)[i_][j_] << i_ + cos(time_consuming_function(10) * j_ + i_);
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// You can also put a CLEF expression in a std::function
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// a function i -> 2*i +1
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std::function<int(int)> f = i_ >> 2 * i_ + 1;
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// a function (i,j) -> 2*i +j
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std::function<double(int, int)> g = var(i_, j_) >> 2 * i_ + j_;
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// checking ...
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std::cout << "f(10) =" << f(10) << " g(1,2) =" << g(1, 2) << std::endl;
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// You can also use a Curry form : h is a function i-> j -> 2*i+ j
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auto h = i_ >> (j_ >> 2 * i_ + j_);
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std::cout << "h(1)(2) = " << h(1)(2) << std::endl;
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// You an also use this to quickly write some lambda, as an alternative syntax to the C++ lambda
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// with e.g. STL algorithms (with the advantage that the function is polymorphic!).
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std::vector<int> v = {0, -1, 2, -3, 4, 5, -6};
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// replace all negative elements (i.e. those for which i -> (i<0) return true), by 0
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std::replace_if(begin(v), end(v), i_ >> (i_ < 0), 0);
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// for non believer, it really worked ...
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for (auto const& x : v) std::cout << x << " ";
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std::cout << std::endl;
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}
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