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

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

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
(C++20)
C library
 
Constrained algorithms
Non-modifying sequence operations
ranges::find_lastranges::find_last_ifranges::find_last_if_not
(C++23)(C++23)(C++23)

Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Numeric operations
(C++23)
Fold operations
Operations on uninitialized storage
Return types
 
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:

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

(C++20)
 finds the last sequence of elements in a certain range
(niebloid)
(C++20)(C++20)(C++20)
 finds the first element satisfying specific criteria
(niebloid)
(C++20)
 searches for a range of elements
(niebloid)
(C++20)
 returns true if one sequence is a subsequence of another
(niebloid)
(C++20)
 determines if an element exists in a partially-ordered range
(niebloid)
(C++23)(C++23)
 checks if the range contains the given element or subrange
(niebloid)
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