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# std::valarray::operator[]

From cppreference.com
< cpp‎ | numeric‎ | valarray

const T& operator[]( size_t pos ) const; | (1) |
T& operator[]( size_t pos ); | (2) |
valarray<T> operator[]( slice slicearr ) const; | (3) |
slice_array<T> operator[]( slice slicearr ); | (4) |
valarray<T> operator[]( const gslice& gslicearr ) const; | (5) |
gslice_array<T> operator[]( const gslice& gslicearr ); | (6) |
valarray<T> operator[]( const valarray<bool>& boolarr ) const; | (7) |
mask_array<T> operator[]( const valarray<bool>& boolarr ); | (8) |
valarray<T> operator[]( const valarray<size_t>& indarr ) const; | (9) |
indirect_array<T> operator[]( const valarray<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.

### [edit] Parameters

| This section is incomplete

### [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.

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