std::valarray<T>::operator[] (3) - Linux Manuals

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


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


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; (1) (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.


 This section is incomplete
 Reason: Explain the parameters

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




The selected elements must exist.


* 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[].


// 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';


  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