std::valarray::operator[]

T                      operator[]( std::size_t pos ) const;

const T&               operator[]( std::size_t pos ) const;

T&                     operator[]( std::size_t pos );

std::valarray<T>       operator[]( std::slice slicearr ) const;

std::slice_array<T>    operator[]( std::slice slicearr );

std::valarray<T>       operator[]( const std::gslice& gslicearr ) const;

std::gslice_array<T>   operator[]( const std::gslice& gslicearr );

std::valarray<T>       operator[]( const valarray<bool>& boolarr ) const;

std::mask_array<T>     operator[]( const valarray<bool>& boolarr );

std::valarray<T>       operator[]( const valarray<std::size_t>& indarr ) const;

std::indirect_array<T> operator[]( const 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

Return value

Exceptions

(none).

Precondition

The selected elements must exist.

Notes

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

• 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++14) 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.

Notes

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

#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';
}

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