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https://github.com/triqs/dft_tools
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102 lines
3.2 KiB
ReStructuredText
102 lines
3.2 KiB
ReStructuredText
.. highlight:: c
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.. _Slicing:
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Element access & partial views
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==================================
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Element access
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--------------------
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* The elements of the arrays can be naturally accessed using the ( ) operator::
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array<T,2> A(10,10); // defines an array
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A(1,2) =10 // element access.
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Partial view, slices
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------------------------
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Various kind of partial views and slices can be made on arrays and matrices.
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* A `partial view` is defined as a view of a restricted portion of the array while
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a `slice` is strictly speaking a partial view of a lower dimension of the original array,
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e.g. a column of a matrix.
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* Partial views uses the ( ) operator, as the evaluation of the array::
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array<T,2> A(10,10); // defines an array
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A ( 1, range(0,2) ) // 1d slice
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A ( 1, range() ) // 1d slice taking all the second dim
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A( range(0,10,2), range(0,10,2)) // a 2d slice viewing every each elements with even coordinates.
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array_view<T,1> SL( A(0,range(0,3))); // naming the view
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auto SL = A(0,range(0,3)); // C++0x with auto [check this]
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* **Return type** :
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* Partial views of array or array_view return an array_view.
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* Partial views of vector or vector_view return an vector_view.
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* 2d partial views of matrix or matrix_view return matrix_view.
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* BUT : (1d) slices of matrix or matrix_view return vector_view.
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* 0d slices of anything are converted to the `value_type` of the array.
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The `range` type
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^^^^^^^^^^^^^^^^^^^^^
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`range` mimics the python `range`. It can be constructed with :
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* no argument : it then takes the whole set of indices in the dimension (like `:` in python) ::
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A(range(), 0) // take the first column of A
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* two arguments to specify a range ::
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A(range (0,3), 0) // means A(0,0), A(1,0), A(2,0)
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.. warning::
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the second element is excluded : range(0,3) is 0,1,2, like in Python.
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* three arguments : a range with a step ::
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A(range(0,4,2), 0) // means A(0,0), A(2,0)
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The `ellipsis` type
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^^^^^^^^^^^^^^^^^^^^^^
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* Ellipsis can be provided in place of `range`, as in python. The type `ellipsis` is similar to range
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except that it is implicitely repeated to as much as necessary.
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* Example ::
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array<long,4> B(2,3,4,5) ;
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B(0, ellipsis(), 3 ) ; // same as B(0, range(),range(), 3 )
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B(0, ellipsis(),2, 3 ) ; // same as B(0, range(), 2, 3 )
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B( ellipsis(),2, 3 ) ; // same as B( range(),range(), 2, 3 )
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* NB : there can be at most one ellipsis per expression (otherwise it would be meaning less).
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* Example of usage :
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Ellipsis are useful to write generic algorithms. For example, imagine that you want to sum
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arrays on their first index ::
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template<typename ArrayType>
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array_view<typename ArrayType::value_type, ArrayType::rank-1> sum0 (ArrayType const & A) {
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array<typename ArrayType::value_type, ArrayType::rank-1> res = A(0,ellipsis());
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for (size_t u =1; u< A.shape()[0]; ++u) res += A(u,ellipsis());
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return res;
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}
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///.....
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array<double,2> A(5,2);
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array<double,3> B(5,2,3);
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std::cout<< sum0(A) << sum0(B) <<std::endl;
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