Difference between revisions of "cpp/algorithm/ranges/max element"

Latest revision as of 14:35, 1 February 2024

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

[edit] Parameters

[edit] Return value

Iterator to the greatest element in the range [first, last). If several elements in the range are equivalent to the greatest element, returns the iterator to the first such element. Returns last if the range is empty (i.e. if first == last).

[edit] Complexity

Exactly max(N - 1, 0) comparisons, where N = ranges::distance(first, last).

[edit] Possible implementation

struct max_element_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == last)
            return last;
 
        auto largest = first;
        while (++first != last)
            if (std::invoke(comp, std::invoke(proj, *largest), std::invoke(proj, *first)))
                largest = first;
        return largest;
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr max_element_fn max_element;

[edit] Example

#include <algorithm>
#include <cmath>
#include <iostream>
 
int main()
{
    namespace ranges = std::ranges;
 
    const auto v = {3, 1, -14, 1, 5, 9, -14, 9};
 
    auto result = ranges::max_element(v.begin(), v.end());
    std::cout << "Max element at pos " << ranges::distance(v.begin(), result) << '\n';
 
    auto abs_compare = [](int a, int b) { return std::abs(a) < std::abs(b); };
    result = ranges::max_element(v, abs_compare);
    std::cout << "Absolute max element at pos "
              << ranges::distance(v.begin(), result) << '\n';
}

Max element at pos 5
Absolute max element at pos 2

[edit] See also