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std::valarray::operator[]

From cppreference.com
< cpp‎ | numeric‎ | valarray
 
 
 
 
(1)
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 ) const;
(5)
std::gslice_array<T>   operator[]( const std::gslice& gslicearr );
(6)
std::valarray<T>       operator[]( const valarray<bool>& boolarr ) const;
(7)
std::mask_array<T>     operator[]( const valarray<bool>& boolarr );
(8)
std::valarray<T>       operator[]( const valarray<std::size_t>& indarr ) const;
(9)
std::indirect_array<T> operator[]( const 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.

Contents

[edit] Parameters

[edit] 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

[edit] Exceptions

(none)

[edit] Precondition

The selected elements must exist.

[edit] 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:

[edit] 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[].

[edit] 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';
}

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