NAME¶

std::valarray::operator[] - std::valarray::operator[]

Synopsis¶

T operator[]( std::size_t pos ) const; (until C++11)
const T& operator[]( std::size_t pos ) const; (since C++11)
T& operator[]( std::size_t pos ); (2)
std::valarray<T> operator[]( std::slice slicearr ) const; (3)
std::slice_array<T> operator[]( std::slice slicearr ); (4)
std::valarray<T> operator[]( const std::gslice& gslicearr ) (5)
const;
std::gslice_array<T> operator[]( const std::gslice& gslicearr (6)
); (1)
std::valarray<T> operator[]( const valarray<bool>& boolarr ) (7)
const;
std::mask_array<T> operator[]( const valarray<bool>& boolarr (8)
);
std::valarray<T> operator[]( const valarray<std::size_t>& (9)
indarr ) const;
std::indirect_array<T> operator[]( const (10)
valarray<std::size_t>& indarr );

Retrieve single elements or portions of the array.

The const overloads that return element sequences create a new std::valarray object.
The non-const overloads return classes holding references to the array elements.

Parameters¶

pos - position of the element to return
slicearr - slice of the elements to return
gslicearr - gslice of the elements to return
boolarr - mask of the elements to return
indarr - indices of the elements to return

Return value¶

1,2) A reference to the corresponding element
3,5,7,9) A std::valarray object containing copies of the selected items
4,6,8,10) The corresponding data structure containing references to the selected
items

Exceptions¶

May throw implementation-defined exceptions.

Precondition¶

The selected elements must exist.

Notes¶

* For proper values of i and j, the following properties are true:

1) (a[i] = q, a[i]) == q

For a non-const a.

2) &a[i+j] == &a[i] + j

This means that valarray elements are adjacent in memory.

3) &a[i] != &b[j]

This holds for every objects a and b that are not aliases of one another.
This means that there are no aliases in the elements and this property can
be used to perform some kinds of optimization.

* References become invalid on resize or when the array is destructed.

For overloads (3,5,7,9), The function can be implemented with the return type
different from std::valarray. In this case, the replacement type has the following
properties:

* All const member functions of std::valarray are provided.
* std::valarray, std::slice_array, std::gslice_array, std::mask_array and
std::indirect_array can be constructed from the replacement type.
* All functions accepting an argument of type const std::valarray&
except begin() and end()
(since C++11) should also accept the replacement type.
* All functions accepting two arguments of type const std::valarray&
should accept every combination of const std::valarray& and the
replacement type.
* The return type does not add more than two levels of template nesting
over the most deeply-nested argument type.

Slice/mask/indirect index accesses do not chain: v[v==n][std::slice(0,5,2)] = x; is
an error because std::mask_array (the type of v[v==n]) does not have operator[].

Example¶

// Run this code

#include <iostream>
#include <valarray>

int main()
{
std::valarray<int> data = {0,1,2,3,4,5,6,7,8,9};

std::cout << "Initial valarray: ";
for(int n: data) std::cout << n << ' ';
std::cout << '\n';

data[data > 5] = -1; // valarray<bool> overload of operator[]
// the type of data>5 is std::valarray<bool>
// the type of data[data>5] is std::mask_array<int>

std::cout << "After v[v>5]=-1: ";
for(std::size_t n = 0; n < data.size(); ++n)
std::cout << data[n] << ' '; // regular operator[]
std::cout << '\n';
}

Output:¶

Initial valarray: 0 1 2 3 4 5 6 7 8 9
After v[v>5]=-1: 0 1 2 3 4 5 -1 -1 -1 -1