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
63 lines
2.0 KiB
ReStructuredText
63 lines
2.0 KiB
ReStructuredText
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.. highlight:: c
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Evaluating CLEF expressions
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===============================
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Forming expressions is nice, but completely useless unless one can *evaluate* them
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by affecting some values to the placeholder and actually *do* the computation.
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Expressions are evaluated with the eval function ::
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eval ( expression , placeholder_1 = value_1, placeholder_2 = value_2, ...)
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The evaluation can be :
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* *complete* : when a value has been affected to every placeholder, so the result is not
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an expression any more, but the result of the computation....
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* *partial* : when some placeholder are still free (no value has been affected to them).
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The result is another expression where the parts that could be computed have been computed.
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Complete evaluation
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--------------------
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.. compileblock::
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#include <triqs/clef.hpp>
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using namespace triqs::clef;
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int main () {
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placeholder<1> x_; placeholder<2> y_;
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std::cout << eval (x_ + 2*y_ , x_=1, y_ =2) << std::endl;
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std::cout << eval (x_ + 2*y_ , y_=2, x_ =1) << std::endl;
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}
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Note that :
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* The order of placeholder does not matter in calling eval.
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* It is an error to put the same placeholder twice.
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* The correct version of eval is found by ADL (Argument Dependent Lookup) in the triqs::clef namespace.
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Partial evaluation
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--------------------
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The evaluation can also be partial, in which case the compiler replaces the placeholder whose value is provided
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and rebuild a new expression tree.
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Example ::
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auto e1 = x_ + 2*y_;
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// return type ---> value
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auto r = eval( e1 , x_ =1); // expression ---> 1 + (2 * y_)
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auto e2 = eval( e1 , x_ =x_ + y_); // expression ---> ((x_ + y_) + (2 * y_))
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auto r = eval( e2, x_ = 1, y_ = 2); // int ---> 7
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During the partial evaluation, all subtrees that can evaluated are evaluated.
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For example ::
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eval(x_ + f(y_) , y_=1); // --> x + f(1)
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In this case, the function f is called by eval.
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