mirror of
https://github.com/triqs/dft_tools
synced 2024-12-27 14:53:39 +01:00
2c542647fd
- change : all objects are by default stored now by reference, not by copy any more. Unless the trait force_copy_in_expr is true. - rvalue refs are moved into the tree - simplifies a lot the writing of lazy method, objects. - added a macro for methods - tests ok. Further check needed to control absence of copies... - improved documentation
124 lines
3.7 KiB
ReStructuredText
124 lines
3.7 KiB
ReStructuredText
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.. highlight:: c
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Transform CLEF expressions into functions
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===============================================
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Clef expressions are **NOT** functions. In short,
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* clef expressions are *evaluated* (the order of argument *does not* matter) ::
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eval( expr, x_=1, y_=2, ...);
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* while functions are *called*, as usual (the order of argument *does* matter !) ::
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f(1,2)
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It is however possible to transform expressions into functions, *as soon as you specify the order of the placeholders*.
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(the opposite is true, if the function accept lazy arguments, cf :ref:`overload_function`).
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make_function
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---------------
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Given any expression with placeholder `x_`, `y_`, `z_`, ..., `make_function`
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transform them into a regular function. If we say ::
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auto f = make_function( clef_expression, placeholder_1, placeholder_2, placeholder_3, ...)
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then f is ::
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a function (x1,x2,x3) --> RESULT
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where RESULT is :
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* the result of the complete evaluation of the expression if the list of placeholder exhausts the placeholders of the expression.
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* otherwise a clef_expression of the remaining placeholders, returning a **function**.
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Short notation with >> operator
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.....................................
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For function of *one* variable, the make_function notation can be simplified into ::
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// same
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auto f = make_function( 2*x_ + y_ + 1, x_);
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auto f = x_ >> 2*x_ + y_ + 1;
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// same
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auto f = make_function ( make_function( 2*x_ + y_ + 1, y_), x_);
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auto f = x_ >> (y_ >> 2*x_ + y_ + 1) ;
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.. warning::
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The notation ::
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`x_` >> `y_` >> expression
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is banned because it conflicts with the standard priority of >>.
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Use parenthesis.
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clef::function
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--------------------------
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The class triqs::clef::function stored a function of a given signature
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It is similar to std::function but
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it can be constructed from an expression and an ordered list of placeholders.
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clef::function can be assigned with the = operator, Cf example below.
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.. note::
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Like std::function, it stores the expression polymorphically, by erasing its type.
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This might lead to some performance penalty in some case, even though tests do not show that at present...
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Examples
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---------
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.. compileblock::
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#include <triqs/clef.hpp>
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#include <iostream>
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using namespace triqs::clef;
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int main() {
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placeholder<0> x_; placeholder<1> y_;
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{ // with one variable
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auto f = make_function(2*x_ + 1, x_);
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std::cout << f(3) << std::endl;
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std::function<double(double)> F(f);
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}
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{ //with two variables
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auto f = make_function(2*x_ + y_ + 1, x_, y_);
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std::cout << f(3,4) << std::endl;
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std::function<double(double,double)> F(f);
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}
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{ // Make a function partially
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auto f = make_function( 2*x_ + y_ + 1, x_);
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// f is a lazy expression expression with placeholder y_, returning a function...
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auto f1 = eval (f, y_=1); // f1 is a function x-> 2*x + 2
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std::cout << f1 (10) << std::endl;
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}
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{ // Currying a function
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//auto f = make_function ( make_function( 2*x_ + y_ + 1, x_), y_);
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auto f = y_ >> ( x_ >> 2*x_ + y_ + 1);
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// f a function y-> x-> 2x+y+1
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// f(y) returns a function x-> 2x+y+1
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auto g = f(3);
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std::cout << g (10) << std::endl;
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}
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{ // playing with clef::function and std::function
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triqs::clef::function<double(double,double)> f2,g2;
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f2(x_,y_) = x_ + y_;
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std::cout << f2(2,3) << std::endl;
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std::function<double(double,double)> sf2 = f2;
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std::cout << sf2(2,3) << std::endl;
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g2(x_,y_) = x_ - y_ + f2(x_,2*y_);
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std::function<double(double)> sf = x_>> 2*x_ + 1;
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std::cout << sf(3) << std::endl;
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}
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}
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