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https://github.com/triqs/dft_tools
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79 lines
1.9 KiB
ReStructuredText
79 lines
1.9 KiB
ReStructuredText
.. highlight:: c
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Functional constructs : map, fold, ...
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========================================================
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map
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--------
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* **Purpose** :
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map promotes any function into an `array function`, acting term by term.
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* **Syntax** :
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If `f` is a function, or a function object (SEE : ref to function object lib)::
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f : ValueType1 --> ValueType2
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Then map(f) is a function::
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ReturnType map(f) ( ArgType const & A)
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where ArgType models the :ref:`HasImmutableArrayInterface` concept
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* with value_type == ValueType1
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and ReturnType models the :ref:`HasImmutableArrayInterface` concept
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* with the same domain as ArgType
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* with value_type == ValueType2
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* Examples::
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double f(int i) { return i*10;}
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int main() {
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auto F = map(f);
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array<int,2> A(2,2);
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array<double,2> B;
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A() =2;
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B = F(A);
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// works also with expressions of course
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C = F( 2*A );
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C = C + 3* F(2*A); // ok that is just an example...
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}
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* Some cases require explicit cast, e.g. for the standard abs function (defined in XXXX), or the compiler does not know which std::abs you are talking about ::
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auto Abs = map( static_cast< double (*)(double)> (std::abs) );
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fold
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-------------------------------------------------
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* **Purpose** :
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fold implement the folding (or reduction) on the array.
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* **Syntax** :
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If A is a type which models the :ref:`HasImmutableArrayInterface` concept
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(e.g. an array<ValueType1,R, Opt> , a matrix, a vector, an expression, ...)
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and `f` is a function, or a function object (SEE : ref to function object lib)::
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ValueType2 f (A::value_type , ValueType2)
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Then fold (f , A, ValueType2 init = ValueType2() ) = f( f( f( ... f( a(0,1), a(0,0)))))
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.. warning::
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The order of traversal is the same as foreach.
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* **Example** : sum is implemented like this ::
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template <class A>
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typename A::value_type sum(A const & a) { return fold ( std::plus<typename A::value_type>()) (a); }
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