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std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not - cppreference.com

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)
(until C++26)		 
template< std::forward_iterator I, std::sentinel_for<I> S,
          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj> >
    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 = {} );
(since C++26) 
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)
(until C++26)		 
template< ranges::forward_range R,
          class Proj = std::identity,
          class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
    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 = {} );
(since C++26) 
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) 
template< /*execution-policy*/ Ep,
          std::random_access_iterator I, std::sized_sentinel_for<I> S,
          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj> >
    requires std::indirect_binary_predicate
                 <ranges::equal_to, std::projected<I, Proj>, const T*>
ranges::subrange<I> find_last( Ep&& policy, I first, S last,
                               const T& value, Proj proj = {} );
(7) (since C++26) 
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
          class Proj = std::identity,
          class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
    requires std::indirect_binary_predicate
                 <ranges::equal_to,
                  std::projected<ranges::iterator_t<R>, Proj>, const T*>
ranges::borrowed_subrange_t<R>
    find_last( Ep&& policy, R&& r, const T& value, Proj proj = {} );
(8) (since C++26) 
template< /*execution-policy*/ Ep,
          std::random_access_iterator I, std::sized_sentinel_for<I> S,
          class Proj = std::identity,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
ranges::subrange<I> find_last_if( Ep&& policy, I first, S last,
                                  Pred pred, Proj proj = {} );
(9) (since C++26) 
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
          class Proj = std::identity,
          std::indirect_unary_predicate
              <std::projected<ranges::iterator_t<R>, Proj>> Pred >
ranges::borrowed_subrange_t<R>
    find_last_if( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
(10) (since C++26) 
template< /*execution-policy*/ Ep,
          std::random_access_iterator I, std::sized_sentinel_for<I> S,
          class Proj = std::identity,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
ranges::subrange<I> find_last_if_not( Ep&& policy, I first, S last,
                                      Pred pred, Proj proj = {} );
(11) (since C++26) 
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
          class Proj = std::identity,
          std::indirect_unary_predicate
              <std::projected<ranges::iterator_t<R>, Proj>> Pred >
ranges::borrowed_subrange_t<R>
    find_last_if_not( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
(12) (since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.

Returns the last element (projected by proj) in the source range [first, last) or r that satisfies specific criteria:

1,2) find_last searches for the last element equal to value.

3,4) find_last_if searches for the last element for which predicate pred returns true.

5,6) find_last_if_not searches for the last element for which predicate pred returns false.

7-12) Same as (1-6), but executed according to policy.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Parameters

first, last - the iterator-sentinel pair defining the source range
r - the source range
value - the target value
pred - the predicate to be applied to the (projected) elements
proj - the projection to be applied to the elements
policy - the execution policy to use

Return value

A subrange from the last element satisfying the condition to the end of the source range, or an empty range if no such element is found.

Complexity

Given N as ranges::distance(first, last) or ranges::distance(r):

1,2) At most N comparisons and applications of proj.

3-6) At most N applications of pred and proj.

7,8) 𝓞(N) comparisons and applications of proj.

9-12) 𝓞(N) applications of pred and proj.

Exceptions

7-12) During the execution process:

  • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
  • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

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 Feature
__cpp_lib_ranges_find_last 202207L (C++23) ranges::find_last,
ranges::find_last_if,
ranges::find_last_if_not
__cpp_lib_algorithm_default_value_type 202403L (C++26) List-initialization for algorithms (1,2)

Possible implementation

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

find_last 
struct find_last_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    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.
        std::optional<I> found;
        for (; first != last; ++first)
            if (std::invoke(proj, *first) == value)
                found = first;
        
        if (!found)
            return {first, first};
        
        return {*found, ranges::next(*found, last)};
    }
    
    template<ranges::forward_range R,
             class Proj = std::identity,
             class T = std::projected_value_t<iterator_t<R>, Proj>>
    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)(ranges::begin(r),
                       ranges::next(ranges::begin(r), ranges::end(r)),
                       value, std::ref(proj));
    }
};

inline constexpr find_last_fn find_last;
find_last_if 
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.
        std::optional<I> found;
        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first)))
                found = first;
        
        if (!found)
            return {first, first};
        
        return {*found, 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)(ranges::begin(r),
                       ranges::next(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 
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.
        std::optional<I> found;
        for (; first != last; ++first)
            if (!std::invoke(pred, std::invoke(proj, *first)))
                found = first;
        
        if (!found)
            return {first, first};
        
        return {*found, 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)(ranges::begin(r),
                       ranges::next(ranges::begin(r), ranges::end(r)),
                       std::ref(pred), std::ref(proj));
    }
};

inline constexpr find_last_if_not_fn find_last_if_not;

Example

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

int main()
{
    namespace ranges = std::ranges;
    
    constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};
    
    {
        constexpr auto i1 = ranges::find_last(v.begin(), v.end(), 3);
        constexpr auto i2 = ranges::find_last(v, 3);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        constexpr auto i1 = ranges::find_last(v.begin(), v.end(), -3);
        constexpr auto i2 = 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 = ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = ranges::find_last_if(v, pred, abs);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == -3; };
        constexpr auto i1 = ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = 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 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = ranges::find_last_if_not(v, pred, abs);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
        constexpr auto i1 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = 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 = ranges::find_last_if(list, cmp_one, &P::first);
    
    std::cout << "The found element and the tail after it are:\n";
    for (P const& e : subrange)
        std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
    std::cout << '\n';
    
#if __cpp_lib_algorithm_default_value_type
    const auto i3 = ranges::find_last(list, {"three", 3}); // (2) C++26
#else
    const auto i3 = ranges::find_last(list, P{"three", 3}); // (2) C++23
#endif
    assert(i3.begin()->first == "three" && i3.begin()->second == 3);
}

Output: 

The found element and the tail after it are:
{"one", 4} {"two", 5} {"three", 6}

See also

finds the last sequence of elements in a certain range
(algorithm function object)[edit]
finds the first element satisfying specific criteria
(algorithm function object)[edit]
searches for the first occurrence of a range of elements
(algorithm function object)[edit]
determines if one sequence is a subsequence of another
(algorithm function object)[edit]
determines if an element exists in a range using binary search
(algorithm function object)[edit]
checks if the range contains the given element or subrange
(algorithm function object)[edit]