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

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< 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.

### Parameters

| 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

### Exceptions

(none)

### 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.

### 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

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

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