std::find_end(3)

✨ NAME ✨

okay so this is the std::find_end(3) page from the C++ Standard Libary manual, and the NAME is literally std::find_end - std::find_end — no notes, no extra lore, just the function being its own icon 💅.

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📚 Synopsis 📚

Defined in header <algorithm>

template< class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,           (1)
                     ForwardIt2 s_first, ForwardIt2 s_last );     (constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ExecutionPolicy&& policy,                    (2) (since C++17)
                     ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last );

template< class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last,           (3)
                     ForwardIt2 s_first, ForwardIt2 s_last,       (constexpr since C++20)
                     BinaryPred p );

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_end( ExecutionPolicy&& policy,                    (4) (since C++17)
                     ForwardIt1 first, ForwardIt1 last,
                     ForwardIt2 s_first, ForwardIt2 s_last,
                     BinaryPred p );

So what does it do? It searches for the last occurrence of the sequence [s_first, s_last) in the range [first, last). Like, it's giving "find the final matching chunk" energy 🔥.

• 1) Elements are compared using operator==.
• 3) Elements are compared using the given binary predicate p.
• 2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if:

• 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)

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🌸 Parameters 🌸

• first, last — the range of elements to examine.
• s_first, s_last — the range of elements to search for.
• policy — the execution policy to use. See execution policy for details.
• p — binary predicate which returns true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

bool pred(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 objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively.

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🦋 Type requirements 🦋

• ForwardIt1 must meet the requirements of LegacyForwardIterator.
• ForwardIt2 must meet the requirements of LegacyForwardIterator.

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🎀 Return value 🎀

Iterator to the beginning of last occurrence of the sequence [s_first, s_last) in range [first, last).

If [s_first, s_last) is empty or if no such sequence is found, last is returned.

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⭐ Complexity ⭐

Given N as std::distance(first1, last1) and S as std::distance(first2, last2):

• 1,2) At most S·(N-S+1) comparisons using operator==.
• 3,4) At most S·(N-S+1) applications of the predicate p.

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💀 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.

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🧚 Possible implementation 🧚

🌟 find_end (1)

template<class ForwardIt1, class ForwardIt2>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last)
{
    if (s_first == s_last)
        return last;

    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}

🌟 find_end (3)

template<class ForwardIt1, class ForwardIt2, class BinaryPred>
constexpr //< since C++20
ForwardIt1 find_end(ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 s_first, ForwardIt2 s_last,
                    BinaryPred p)
{
    if (s_first == s_last)
        return last;

    ForwardIt1 result = last;
    while (true)
    {
        ForwardIt1 new_result = std::search(first, last, s_first, s_last, p);
        if (new_result == last)
            break;
        else
        {
            result = new_result;
            first = result;
            ++first;
        }
    }
    return result;
}

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🍓 Example 🍓

// Run this code

#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>

auto print_result = [](auto result, const auto& v)
{
    result == v.end()
        ? std::cout << "Sequence not found\n"
        : std::cout << "Last occurrence is at: " << std::distance(v.begin(), result)
                    << '\n';
};

int main()
{
    const auto v = {1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4};

    for (auto const& x : {std::array{1, 2, 3}, {4, 5, 6}})
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end()); // overload (1)
        print_result(iter, v);
    }

    for (auto const& x : {std::array{-1, -2, -3}, {-4, -5, -6}})
    {
        auto iter = std::find_end(v.begin(), v.end(), x.begin(), x.end(), // overload (3)
                                  [](int x, int y)
                                  {
                                      return std::abs(x) == std::abs(y);
                                  });
        print_result(iter, v);
    }
}

🌈 Output:

Last occurrence is at: 8
Sequence not found
Last occurrence is at: 8
Sequence not found

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👑 Defect reports 👑

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 1205 — applied to C++98. Behavior as published: the return value was unclear if [s_first, s_last) is empty. Correct behavior: returns last in this case.
• LWG 2150 — applied to C++98. Behavior as published: the condition of “sequence occurence” was incorrect. Correct behavior: corrected.

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🫶 See also 🫶

• search — searches for a range of elements (function template).
• includes — returns true if one sequence is a subsequence of another (function template).
• adjacent_find — finds the first two adjacent items that are equal (or satisfy a given predicate) (function template).
• find — finds the first element satisfying specific criteria (function template).
• find_if — finds the first element satisfying specific criteria (function template).
• find_if_not (C++11) — finds the first element satisfying specific criteria (function template).
• find_first_of — searches for any one of a set of elements (function template).
• search_n — searches a range for a number of consecutive copies of an element (function template).
• ranges::find_end (C++20) — finds the last sequence of elements in a certain range (niebloid).

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okay that's the whole page, bestie! source/version and date from the footer: http://cppreference.com, 2024.06.10, for std::find_end(3) — mwah 💋✨