# std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> find_last( I first, S last, const T& value, Proj proj = {} ); | (1) | (since C++23)
template< ranges::forward_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_subrange_t<R> find_last( R&& r, const T& value, Proj proj = {} ); | (2) | (since C++23)
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr ranges::subrange<I> find_last_if( I first, S last, Pred pred, Proj proj = {} ); | (3) | (since C++23)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_subrange_t<R> find_last_if( R&& r, Pred pred, Proj proj = {} ); | (4) | (since C++23)
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr ranges::subrange<I> find_last_if_not( I first, S last, Pred pred, Proj proj = {} ); | (5) | (since C++23)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_subrange_t<R> find_last_if_not( R&& r, Pred pred, Proj proj = {} ); | (6) | (since C++23)

Returns the last element in the range `[`first`,` last`)` that satisfies specific criteria:

1) `find_last` searches for an element equal to value.
3) `find_last_if` searches for the last element in the range `[`first`,` last`)` for which predicate pred returns true.
5) `find_last_if_not` searches for the last element in the range `[`first`,` last`)` for which predicate pred returns false.
2,4,6) Same as (1,3,5), 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:

- 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.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of the elements to examine
value | - | value to compare the elements to
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

1,2,3) Let `i` be the last iterator in the range `[`first`,` last`)` for which `E` is true. Returns ranges::subrange<I>{i, last}, or ranges::subrange<I>{last, last} if no such iterator is found.
2,4,6) Same as (1,2,3) but the return type is ranges::borrowed_subrange_t<I>.

### Complexity

At most last - first applications of the predicate and projection.

### Notes

`ranges::find_last`, `ranges::find_last_if`, `ranges::find_last_if_not` have better efficiency on common implementations if I models `bidirectional_iterator` or (better) `random_access_iterator`.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_ranges_find_last` | 202207L | (C++23) | `ranges::find_last`, `ranges::find_last_if`, `ranges::find_last_if_not`

### Possible implementation

These implementations only show the slower algorithm used when I models `forward_iterator`.

find_last (1-2)
    struct find_last_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S,
                 class T, class Proj = std::identity>
        requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
                                                const T*>
        constexpr ranges::subrange<I>
            operator()(I first, S last, const T &value, Proj proj = {}) const
        {
            // Note: if I is mere forward_iterator, we may only go from begin to end.
            I found {};
            for (; first != last; ++first)
                if (std::invoke(proj, *first) == value)
                    found = first;

            if (found == I {})
                return {first, first};

            return {found, std::ranges::next(found, last)};
        }

        template<ranges::forward_range R, class T, class Proj = std::identity>
        requires std::indirect_binary_predicate<ranges::equal_to,
                                                std::projected<ranges::iterator_t<R>, Proj>,
                                                const T*>
        constexpr ranges::borrowed_subrange_t<R>
            operator()(R&& r, const T &value, Proj proj = {}) const
        {
            return this->operator()(ranges::begin(r), ranges::end(r), value, std::ref(proj));
        }
    };

    inline constexpr find_last_fn find_last;
find_last_if (3-4)
    struct find_last_if_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr ranges::subrange<I>
            operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            // Note: if I is mere forward_iterator, we may only go from begin to end.
            I found {};
            for (; first != last; ++first)
                if (std::invoke(pred, std::invoke(proj, *first)))
                    found = first;

            if (found == I {})
                return {first, first};

            return {found, std::ranges::next(found, last)};
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
                     Pred>
        constexpr ranges::borrowed_subrange_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return this->operator()(ranges::begin(r), ranges::end(r),
                                    std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr find_last_if_fn find_last_if;
find_last_if_not (5-6)
    struct find_last_if_not_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr ranges::subrange<I>
            operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            // Note: if I is mere forward_iterator, we may only go from begin to end.
            I found {};
            for (; first != last; ++first)
                if (!std::invoke(pred, std::invoke(proj, *first)))
                    found = first;

            if (found == I {})
                return {first, first};

            return {found, std::ranges::next(found, last)};
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
                     Pred>
        constexpr ranges::borrowed_subrange_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return this->operator()(ranges::begin(r), ranges::end(r),
                                    std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr find_last_if_not_fn find_last_if_not;

### Example

A link to test: Compiler Explorer/g++-trunk

Run this code

    #include <algorithm>
    #include <forward_list>
    #include <iomanip>
    #include <iostream>
    #include <string_view>

    int main()
    {
        constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};

        {
            constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), 3);
            constexpr auto i2 = std::ranges::find_last(v, 3);
            static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
            static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
        }
        {
            constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), -3);
            constexpr auto i2 = std::ranges::find_last(v, -3);
            static_assert(i1.begin() == v.end());
            static_assert(i2.begin() == v.end());
        }

        auto abs = [](int x) { return x < 0 ? -x : x; };

        {
            auto pred = [](int x) { return x == 3; };
            constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
            constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
            static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
            static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
        }
        {
            auto pred = [](int x) { return x == -3; };
            constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
            constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
            static_assert(i1.begin() == v.end());
            static_assert(i2.begin() == v.end());
        }

        {
            auto pred = [](int x) { return x == 1 or x == 2; };
            constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
            constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
            static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
            static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
        }
        {
            auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
            constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
            constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
            static_assert(i1.begin() == v.end());
            static_assert(i2.begin() == v.end());
        }

        using P = std::pair<std::string_view, int>;
        std::forward_list<P> list
        {
            {"one", 1}, {"two", 2}, {"three", 3},
            {"one", 4}, {"two", 5}, {"three", 6},
        };
        auto cmp_one = [](const std::string_view &s) { return s == "one"; };

        // find latest element that satisfy the comparator, and projecting pair::first
        const auto subrange = std::ranges::find_last_if(list, cmp_one, &P::first);

        // print the found element and the "tail" after it
        for (P const& e : subrange)
            std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
        std::cout << '\n';
    }

Output:

    {"one", 4} {"two", 5} {"three", 6}

### See also

ranges::find_end (C++20) | finds the last sequence of elements in a certain range (niebloid)
ranges::find ranges::find_if ranges::find_if_not (C++20)(C++20)(C++20) | finds the first element satisfying specific criteria (niebloid)
ranges::search (C++20) | searches for a range of elements (niebloid)
ranges::includes (C++20) | returns true if one sequence is a subsequence of another (niebloid)
ranges::binary_search (C++20) | determines if an element exists in a partially-ordered range (niebloid)
ranges::contains ranges::contains_subrange (C++23)(C++23) | checks if the range contains the given element or subrange (niebloid)

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