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std::minmax_element

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > std::pair<ForwardIt,ForwardIt> minmax_element( ForwardIt first, ForwardIt last );		(since C++11) (until C++17)
template< class ForwardIt > constexpr std::pair<ForwardIt,ForwardIt> minmax_element( ForwardIt first, ForwardIt last );		(since C++17)
template< class ExecutionPolicy, class ForwardIt > std::pair<ForwardIt,ForwardIt> minmax_element( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since C++17)
	(3)
template< class ForwardIt, class Compare > std::pair<ForwardIt,ForwardIt> minmax_element( ForwardIt first, ForwardIt last, Compare comp );		(since C++11) (until C++17)
template< class ForwardIt, class Compare > constexpr std::pair<ForwardIt,ForwardIt> minmax_element( ForwardIt first, ForwardIt last, Compare comp );		(since C++17)
template< class ExecutionPolicy, class ForwardIt, class Compare > std::pair<ForwardIt,ForwardIt> minmax_element( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );	(4)	(since C++17)

Finds the smallest and greatest element in the range [first, last).

1) Elements are compared using operator<.

3) Elements are compared using the given binary comparison function comp.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first, last	-	forward iterators defining the range to examine
policy	-	the execution policy to use. See execution policy for details.
cmp	-	comparison function object (i.e. an object that satisfies the requirements of Compare) which returns `true` if if `a` is less* than `*b`. The signature of the comparison function should be equivalent to the following: `bool cmp(const Type1 &a, const Type2 &b);` While the signature does not need to have `const &`, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1 &` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy (since C++11)). The types `Type1` and `Type2` must be such that an object of type `ForwardIt` can be dereferenced and then implicitly converted to both of them.
Type requirements
-`ForwardIt` must meet the requirements of LegacyForwardIterator.

Return value

a pair consisting of an iterator to the smallest element as the first element and an iterator to the greatest element as the second. Returns std::make_pair(first, first) if the range is empty. If several elements are equivalent to the smallest element, the iterator to the first such element is returned. If several elements are equivalent to the largest element, the iterator to the last such element is returned.

Complexity

At most max(floor((3/2)*(N−1)), 0) applications of the predicate, where N = std::distance(first, last).

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes

This algorithm is different from std::make_pair(std::min_element(), std::max_element()), not only in efficiency, but also in that this algorithm finds the last biggest element while std::max_element finds the first biggest element.

Possible implementation

First version
template<class ForwardIt> std::pair<ForwardIt, ForwardIt> minmax_element(ForwardIt first, ForwardIt last) { using value_type = typename std::iterator_traits<ForwardIt>::value_type; return std::minmax_element(first, last, std::less<value_type>()); }
Second version
template<class ForwardIt, class Compare> std::pair<ForwardIt, ForwardIt> minmax_element(ForwardIt first, ForwardIt last, Compare comp) { auto min = first, max = first; if (first == last \|\| ++first == last) return {min, max}; if (comp(first, min)) { min = first; } else { max = first; } while (++first != last) { auto i = first; if (++first == last) { if (comp(i, min)) min = i; else if (!(comp(i, max))) max = i; break; } else { if (comp(first, i)) { if (comp(first, min)) min = first; if (!(comp(i, max))) max = i; } else { if (comp(i, min)) min = i; if (!(comp(first, max))) max = first; } } } return {min, max}; }

First version

template<class ForwardIt>
std::pair<ForwardIt, ForwardIt> 
    minmax_element(ForwardIt first, ForwardIt last)
{
    using value_type = typename std::iterator_traits<ForwardIt>::value_type;
    return std::minmax_element(first, last, std::less<value_type>());
}

Second version

template<class ForwardIt, class Compare>
std::pair<ForwardIt, ForwardIt> 
    minmax_element(ForwardIt first, ForwardIt last, Compare comp)
{
    auto min = first, max = first;
 
    if (first == last || ++first == last)
        return {min, max};
 
    if (comp(*first, *min)) {
        min = first;
    } else {
        max = first;
    }
 
    while (++first != last) {
        auto i = first;
        if (++first == last) {
            if (comp(*i, *min)) min = i;
            else if (!(comp(*i, *max))) max = i;
            break;
        } else {
            if (comp(*first, *i)) {
                if (comp(*first, *min)) min = first;
                if (!(comp(*i, *max))) max = i;
            } else {
                if (comp(*i, *min)) min = i;
                if (!(comp(*first, *max))) max = first;
            }
        }
    }
    return {min, max};
}

Example

#include <algorithm>
#include <iostream>
#include <vector>
 
int main() {
    const auto v = { 3, 9, 1, 4, 2, 5, 9 };
    const auto [min, max] = std::minmax_element(begin(v), end(v));
 
    std::cout << "min = " << *min << ", max = " << *max << '\n';
}

Output:

min = 1, max = 9

min_element	returns the smallest element in a range (function template)
max_element	returns the largest element in a range (function template)