# operator==,!=,<,<=,>,>=(std::valarray)

From cppreference.com
< cpp‎ | numeric‎ | valarray

template< class T >

std::valarray <bool> operator==( const std::valarray <T>& lhs, const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator!=( const std::valarray <T>& lhs, const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator<( const std::valarray <T>& lhs, const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator<=( const std::valarray <T>& lhs, const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator>( const std::valarray <T>& lhs, const std::valarray <T>& rhs );
template< class T >

std::valarray <bool> operator>=( const std::valarray <T>& lhs, const std::valarray <T>& rhs );

(1)

template< class T >

std::valarray <bool> operator==( const typename std::valarray <T>::value_type & lhsv,
const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator!=( const typename std::valarray <T>::value_type & lhsv,
const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator< ( const typename std::valarray <T>::value_type & lhsv,
const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator<=( const typename std::valarray <T>::value_type & lhsv,
const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator> ( const typename std::valarray <T>::value_type & lhsv,
const std::valarray <T>& rhs );
template< class T >
std::valarray <bool> operator>=( const typename std::valarray <T>::value_type & lhsv,

const std::valarray <T>& rhs );

(2)

template< class T >

std::valarray <bool> operator==( const std::valarray <T>& lhs,
const typename std::valarray <T>::value_type & rhsv );
template< class T >
std::valarray <bool> operator!=( const std::valarray <T>& lhs,
const typename std::valarray <T>::value_type & rhsv );
template< class T >
std::valarray <bool> operator< ( const std::valarray <T>& lhs,
const typename std::valarray <T>::value_type & rhsv );
template< class T >
std::valarray <bool> operator<=( const std::valarray <T>& lhs,
const typename std::valarray <T>::value_type & rhsv );
template< class T >
std::valarray <bool> operator> ( const std::valarray <T>& lhs,
const typename std::valarray <T>::value_type & rhsv );
template< class T >
std::valarray <bool> operator>=( const std::valarray <T>& lhs,

const typename std::valarray <T>::value_type & rhsv );

(3)

Compares each value within the numeric array with another value.

1) Returns a numeric array of bool containing elements each of which is obtained by applying the indicated comparison operator to the corresponding values of `lhs` and `rhs`

The behavior is undefined if size() != v.size()

2) Returns a numeric array of bool containing elements each of which is obtained by applying the indicated comparison operator to `lhsv` and the corresponding value of `rhs` .
3) Returns a numeric array of bool containing elements each of which is obtained by applying the indicated comparison operator to the corresponding value of `lhs` and `rhsv`.

### Parameters

lhs, rhs | - | numeric arrays to compare
lhsv, rhsv | - | values to compare to each element within a numeric array

### Return value

A numeric array of bool containing comparison results of corresponding elements.

### Exceptions

(none)

### Notes

Each of the operators can only be instantiated if the following requirements are met:

  - The indicated operator can be applied to type `T`
  - The result value can be unambiguously converted to `bool`.

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.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3074 | C++98 | `T` is deduced from both the scalar and the `valarray` for (2-3), disallowing mixed-type calls | only deduce `T` from the `valarray`

### Example

Run this code

    #include <iostream>
    #include <valarray>

    int main()
    {
        // zero all negatives in a valarray
        std::valarray<int> v = {1, -1, 0, -3, 10, -1, -2};
        std::cout << "Before: ";
        for(auto n: v) std::cout << n << ' '; std::cout << '\n';
        v[v < 0] = 0;
        std::cout << "After: ";
        for(auto n: v) std::cout << n << ' '; std::cout << '\n';

        // convert the valarray<bool> result of == to a single bool
        std::valarray<int> a = {1,2,3};
        std::valarray<int> b = {2,4,6};

        std::cout << "2*a == b is " << std::boolalpha
                  << (2*a == b).min() << '\n';

    }

Output:

    Before: 1 -1 0 -3 10 -1 -2
    After: 1 0 0 0 10 0 0
    2*a == b is true

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