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dft_tools/doc/reference/c++/clef/expressions_eval.rst
Olivier Parcollet 3fe400d34c doc : split c++ code from rst
- examples split from the rst file using a python script (split_code).
- Final result for the doc is unchanged.
- examples are compiled and tested with the other tests.
- examples' code have been clang-formatted, with triqs style.
- doc compiles much faster, and with the same options as the rest of the
  test.
- examples are added as tests, so they are run by make test, as simple C
  tests.
- done for the tutorials and the reference.
- autocompile removed (changed into triqs_example directive).
- add triqs_example :
   - make a literal include of the source code.
   - runs the compiled example
   - add, as before, the result to the source code in the doc.
- added the script split_code, used to make the changes automatically,
  maybe for later reuse. (in _tools)
2014-05-31 23:00:16 +02:00

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.. highlight:: c
Evaluating CLEF expressions
===============================
Forming expressions is nice, but completely useless unless one can *evaluate* them
by affecting some values to the placeholder and actually *do* the computation.
Expressions are evaluated with the eval function ::
eval ( expression , placeholder_1 = value_1, placeholder_2 = value_2, ...)
The evaluation can be :
* *complete* : when a value has been affected to every placeholder, so the result is not
an expression any more, but the result of the computation....
* *partial* : when some placeholder are still free (no value has been affected to them).
The result is another expression where the parts that could be computed have been computed.
Complete evaluation
--------------------
.. triqs_example:: ./expressions_eval_0.cpp
Note that :
* The order of placeholder does not matter in calling eval.
* It is an error to put the same placeholder twice.
* The correct version of eval is found by ADL (Argument Dependent Lookup) in the triqs::clef namespace.
Partial evaluation
--------------------
The evaluation can also be partial, in which case the compiler replaces the placeholder whose value is provided
and rebuild a new expression tree.
Example ::
auto e1 = x_ + 2*y_;
// return type ---> value
auto r = eval( e1 , x_ =1); // expression ---> 1 + (2 * y_)
auto e2 = eval( e1 , x_ =x_ + y_); // expression ---> ((x_ + y_) + (2 * y_))
auto r = eval( e2, x_ = 1, y_ = 2); // int ---> 7
During the partial evaluation, all subtrees that can evaluated are evaluated.
For example ::
eval(x_ + f(y_) , y_=1); // --> x + f(1)
In this case, the function f is called by eval.