# xindex_view¶

Defined in xtensor/xindex_view.hpp

template<class CT, class I>
class xindex_view : public xt::xview_semantic<xindex_view<CT, I>>, public xt::xiterable<xindex_view<CT, I>>, public extension::xindex_view_base_t<CT, I>

View of an xexpression from vector of indices.

The xindex_view class implements a flat (1D) view into a multidimensional xexpression yielding the values at the indices of the index array. xindex_view is not meant to be used directly, but only with the index_view and filter helper functions.

See
Template Parameters
• CT: the closure type of the xexpression type underlying this view

• I: the index array type of the view

Extended copy semantic

template<class E>
auto operator=(const xexpression<E> &e)

The extended assignment operator.

Constructor

template<class CTA, class I2>
xindex_view(CTA &&e, I2 &&indices)

Constructs an xindex_view, selecting the indices specified by indices.

The resulting xexpression has a 1D shape with a length of n for n indices.

Parameters
• e: the underlying xexpression for this view

• indices: the indices to select

Size and shape

auto size() const

Returns the size of the xindex_view.

auto dimension() const

Returns the number of dimensions of the xindex_view.

auto shape() const

Returns the shape of the xindex_view.

auto shape(size_type index) const

Returns the i-th dimension of the expression.

Data

template<class T>
void fill(const T &value)

Fills the view with the given value.

Parameters
• value: the value to fill the view with.

auto operator()(size_type idx = size_type(0))

Returns a reference to the element at the specified position in the xindex_view.

Parameters
• idx: index specifying the position in the index_view. More indices may be provided, only the last one will be used.

auto unchecked(size_type idx)

Returns a reference to the element at the specified position in the xindex_view.

Parameters
• idx: index specifying the position in the index_view.

auto operator()(size_type idx = size_type(0)) const

Returns a constant reference to the element at the specified position in the xindex_view.

Parameters
• idx: index specifying the position in the index_view. More indices may be provided, only the last one will be used.

auto unchecked(size_type idx) const

Returns a constant reference to the element at the specified position in the xindex_view.

Parameters
• idx: index specifying the position in the index_view.

auto expression()

Returns a reference to the underlying expression of the view.

auto expression() const

Returns a constant reference to the underlying expression of the view.

template<class S>
auto operator[](const S &index)

Returns a reference to the element at the specified position in the container.

Parameters
• index: a sequence of indices specifying the position in the container. Indices must be unsigned integers, the number of indices in the list should be equal or greater than the number of dimensions of the container.

template<class S>
auto operator[](const S &index) const

Returns a constant reference to the element at the specified position in the container.

Parameters
• index: a sequence of indices specifying the position in the container. Indices must be unsigned integers, the number of indices in the list should be equal or greater than the number of dimensions of the container.

template<class It>
auto element(It first, It)

Returns a reference to the element at the specified position in the xindex_view.

Parameters
• first: iterator starting the sequence of indices The number of indices in the sequence should be equal to or greater 1.

template<class It>
auto element(It first, It) const

Returns a reference to the element at the specified position in the xindex_view.

Parameters
• first: iterator starting the sequence of indices The number of indices in the sequence should be equal to or greater 1.

template<class O>
bool broadcast_shape(O &shape, bool reuse_cache = false) const

Broadcast the shape of the xindex_view to the specified parameter.

Return

a boolean indicating whether the broadcasting is trivial

Parameters
• shape: the result shape

• reuse_cache: parameter for internal optimization

template<class O>
bool has_linear_assign(const O&) const

Checks whether the xindex_view can be linearly assigned to an expression with the specified strides.

Return

a boolean indicating whether a linear assign is possible

template<class ECT, class CCT>
class xfiltration

Filter of a xexpression for fast scalar assign.

The xfiltration class implements a lazy filtration of a multidimentional xexpression, optimized for scalar and computed scalar assignments. Actually, the xfiltration class IS NOT an xexpression and the scalar and computed scalar assignments are the only method it provides. The filtering condition is not evaluated until the filtration is assigned.

xfiltration is not meant to be used directly, but only with the filtration helper function.

See
Template Parameters
• ECT: the closure type of the xexpression type underlying this filtration

• CCR: the closure type of the filtering xexpression type

Extended copy semantic

template<class E>
auto operator=(const E &e)

Assigns the scalar e to *this.

Return

a reference to \ *this.

Parameters
• e: the scalar to assign.

Constructor

template<class ECTA, class CCTA>
xfiltration(ECTA &&e, CCTA &&condition)

Constructs a xfiltration on the given expression e, selecting the elements matching the specified condition.

Parameters

Computed assignement

template<class E>
auto operator+=(const E &e)

Adds the scalar e to *this.

Return

a reference to *this.

Parameters
• e: the scalar to add.

template<class E>
auto operator-=(const E &e)

Subtracts the scalar e from *this.

Return

a reference to *this.

Parameters
• e: the scalar to subtract.

template<class E>
auto operator*=(const E &e)

Multiplies *this with the scalar e.

Return

a reference to *this.

Parameters
• e: the scalar involved in the operation.

template<class E>
auto operator/=(const E &e)

Divides *this by the scalar e.

Return

a reference to *this.

Parameters
• e: the scalar involved in the operation.

template<class E>
auto operator%=(const E &e)

Computes the remainder of *this after division by the scalar e.

Return

a reference to *this.

Parameters
• e: the scalar involved in the operation.

template<class E, class I>
auto xt::index_view(E &&e, I &&indices)

creates an indexview from a container of indices.

Returns a 1D view with the elements at indices selected.

xarray<double> a = {{1,5,3}, {4,5,6}};
b = index_view(a, {{0, 0}, {1, 0}, {1, 1}});
std::cout << b << std::endl; // {1, 4, 5}
b += 100;
std::cout << a << std::endl; // {{101, 5, 3}, {104, 105, 6}}

Parameters
• e: the underlying xexpression

• indices: the indices to select

template<layout_type L = xt::layout_type::row_major, class E, class O>
auto xt::filter(E &&e, O &&condition)

creates a view into e filtered by condition.

Returns a 1D view with the elements selected where condition evaluates to true. This is equivalent to

{index_view(e, argwhere(condition));}

The returned view is not optimal if you just want to assign a scalar to the filtered elements. In that case, you should consider using the filtration function instead.

xarray<double> a = {{1,5,3}, {4,5,6}};
b = filter(a, a >= 5);
std::cout << b << std::endl; // {5, 5, 6}

Template Parameters
• L: the traversal order

Parameters
• e: the underlying xexpression

• condition: xexpression with shape of e which selects indices

See

template<class E, class C>
auto xt::filtration(E &&e, C &&condition)

creates a filtration of e filtered by condition.

Returns a lazy filtration optimized for scalar assignment. Actually, scalar assignment and computed scalar assignments are the only available methods of the filtration, the filtration IS NOT an xexpression.

xarray<double> a = {{1,5,3}, {4,5,6}};
filtration(a, a >= 5) += 2;
std::cout << a << std::endl; // {{1, 7, 3}, {4, 7, 8}}

Parameters