std::valarray::operator[]

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std::valarray

Member functions
valarray::valarray valarray::~valarray valarray::operator= valarray::operator[] valarray::swap valarray::size valarray::resize valarray::sum valarray::min valarray::max valarray::shift valarray::cshift valarray::apply	valarray::operator+valarray::operator-valarray::operator~valarray::operator! valarray::operator+=valarray::operator-=valarray::operator*=valarray::operator/=valarray::operator%=valarray::operator&=valarray::operator|=valarray::operator^=valarray::operator<<=valarray::operator>>=
Non-member functions
swap (C++11) begin (C++11) end (C++11) abs exp log log10 pow sqrt sin cos tan asin acos atan atan2 sinh cosh tanh	operator*operator/operator%operator+operator-operator^operator&operator|operator<<operator>>operator&&operator|| operator==operator!=operator<operator>operator<=operator>=
Helper classes
slice_array gslice_array indirect_array	slice gslice mask_array
Deduction guides(C++17)

	(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.

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