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std::ranges::max_element

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
ranges::max_element
Permutations
 
Defined in header <algorithm>
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 max_element( I first, S last, Comp comp = {}, Proj proj = {} );
(1) (since C++20)
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>

max_element( R&& r, Comp comp = {}, Proj proj = {} );
(2) (since C++20)
1) Finds the greatest element in the range [first, last).
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

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

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

Contents

[edit] Parameters

first, last - iterator-sentinel pair denoting the range to examine
r - the range to examine
comp - comparison to apply to the projected elements
proj - projection to apply to the elements

[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.

[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;
    ++first;
    for (; first != last; ++first) {
        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 <iostream>
#include <vector>
#include <cmath>
 
static bool abs_compare(int a, int b)
{
    return (std::abs(a) < std::abs(b));
}
 
int main()
{
    std::vector<int> v{ 3, 1, -14, 1, 5, 9 };
 
    namespace ranges = std::ranges;
    auto result = ranges::max_element(v.begin(), v.end());
    std::cout << "max element at: " << ranges::distance(v.begin(), result) << '\n';
 
    result = ranges::max_element(v, abs_compare);
    std::cout << "max element (absolute) at: " << ranges::distance(v.begin(), result) << '\n';
}

Output:

max element at: 5
max element (absolute) at: 2

[edit] See also

returns the smallest element in a range
(niebloid) [edit]
returns the smallest and the largest elements in a range
(niebloid) [edit]
returns the greater of the given values
(niebloid) [edit]
returns the largest element in a range
(function template) [edit]