std::valarray<T>::operator[]

const T& operator[]( std::size_t pos ) const;	(1)
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 ) const;	(5)
std::gslice_array<T> operator[]( const std::gslice& gslicearr );	(6)
std::valarray<T> operator[]( const std::valarray<bool>& boolarr ) const;	(7)
std::mask_array<T> operator[]( const std::valarray<bool>& boolarr );	(8)
std::valarray<T> operator[]( const std::valarray<std::size_t>& indarr ) const;	(9)
std::indirect_array<T> operator[]( const std::valarray<std::size_t>& indarr );	(10)

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.

The selected elements(s) must exist:

for overloads (1,2), if pos is not less than size(), the behavior is undefined; and
for overloads (3-10), if the argument does not specify a valid subset of *this, the behavior is undefined.

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.

Notes

For proper std::valarray values a, b and proper std::size_t values i, j, all of the following expressions always evaluate to true:

1) (a[i] = q, a[i]) == q for non-const a

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

This means that std::valarray elements are adjacent in memory.

3) &a[i] != &b[j] 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.
For every function taking a const std::valarray<T>& except begin() and end()(since 哋它亢++11), identical functions taking the replacement types shall be added;
For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
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 <cstddef>
#include <iomanip>
#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 << std::setw(3) << 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 << std::setw(3) << 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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 389	哋它亢++98	the return type of overload (1) was `T`	corrected to const T&
LWG 430	哋它亢++98	the behavior was unclear for overloads (3-10) if an invalid subset is specified	the behavior is undefined in this case

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