std::minmax_element( ) algorithm

od C++17

// (1)
template< class ForwardIt >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class Compare >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ForwardIt first, ForwardIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class ForwardIt >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class Compare >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ExecutionPolicy&& policy,
                    ForwardIt first, ForwardIt last, Compare comp );

od C++11

// (1)
template< class ForwardIt >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class Compare >
std::pair<ForwardIt, ForwardIt>
    minmax_element( ForwardIt first, ForwardIt last, Compare comp );

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

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

Parameters

first last	The range to find the greatest and smallest value in.
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 *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 (od C++11)). The types Type1 and Type2 must be such that an object of type ForwardIt can be implicitly converted to both of them.

Type requirements

ForwardIt

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

Given N as std::distance(first, last):

At most max(floor( (3 / 2) * (N − 1) ), 0) applications of the predicate.

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.

Possible implementation

minmax_element (1)

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>());
}

minmax_element (2)

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};
}

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 (in terms of iterators).

Examples

Main.cpp

#include <algorithm>
#include <iostream>

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';
}

Possible output

min = 1, max = 9