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std::max_element

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
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(C++11)
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Sorting and related operations
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max_element

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Defined in header <algorithm>
(1)
template< class ForwardIt >
ForwardIt max_element( ForwardIt first, ForwardIt last );
(until C++17)
template< class ForwardIt >
constexpr ForwardIt max_element( ForwardIt first, ForwardIt last );
(since C++17)
template< class ExecutionPolicy, class ForwardIt >

ForwardIt max_element( ExecutionPolicy&& policy,

                       ForwardIt first, ForwardIt last );
(2) (since C++17)
(3)
template< class ForwardIt, class Compare >
ForwardIt max_element( ForwardIt first, ForwardIt last, Compare comp );
(until C++17)
template< class ForwardIt, class Compare >

constexpr ForwardIt max_element( ForwardIt first, ForwardIt last,

                                 Compare comp );
(since C++17)
template< class ExecutionPolicy, class ForwardIt, class Compare >

ForwardIt max_element( ExecutionPolicy&& policy,

                       ForwardIt first, ForwardIt last, Compare comp );
(4) (since C++17)

Finds the greatest element in the range [firstlast).

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.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - forward iterators defining the range to examine
policy - the execution policy to use. See execution policy for details.
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second.

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.

[edit] Return value

Iterator to the greatest element in the range [firstlast). 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 = std::distance(first, last).

[edit] 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.

[edit] Possible implementation

max_element (1)
template<class ForwardIt>
ForwardIt max_element(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return last;
 
    ForwardIt largest = first;
 
    while (++first != last)
        if (*largest < *first)
            largest = first;
 
    return largest;
}
max_element (3)
template<class ForwardIt, class Compare>
ForwardIt max_element(ForwardIt first, ForwardIt last, Compare comp)
{
    if (first == last)
        return last;
 
    ForwardIt largest = first;
 
    while(++first != last)
        if (comp(*largest, *first))
            largest = first;
 
    return largest;
}

[edit] Example

#include <algorithm>
#include <cmath>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{3, 1, -14, 1, 5, 9, -14, 9};
    std::vector<int>::iterator result;
 
    result = std::max_element(v.begin(), v.end());
    std::cout << "Max element found at index "
              << std::distance(v.begin(), result)
              << " has value " << *result << '\n';
 
    result = std::max_element(v.begin(), v.end(), [](int a, int b)
    {
        return std::abs(a) < std::abs(b);
    });
    std::cout << "Absolute max element found at index "
              << std::distance(v.begin(), result)
              << " has value " << *result << '\n';
}

Output:

Max element found at index 5 has value 9
Absolute max element found at index 2 has value -14

[edit] 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 212 C++98 the return value was not specified if first == last returns last in this case

[edit] See also

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