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Difference between revisions of "cpp/algorithm/ranges/find last"

From cppreference.com
< cpp‎ | algorithm‎ | ranges
m (→‎Notes: <br>.)
(Possible implementation update.)
 
(4 intermediate revisions by 3 users not shown)
Line 8: Line 8:
 
           class T,
 
           class T,
 
           class Proj = std::identity >
 
           class Proj = std::identity >
−
requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
+
requires std::indirect_binary_predicate
−
                                        const T*>
+
            <ranges::equal_to, std::projected<I, Proj>, const T*>
 
constexpr ranges::subrange<I>
 
constexpr ranges::subrange<I>
 
     find_last( I first, S last, const T& value, Proj proj = {} );
 
     find_last( I first, S last, const T& value, Proj proj = {} );
Line 16: Line 16:
 
           class Proj = std::identity,
 
           class Proj = std::identity,
 
           class T = std::projected_value_t<I, Proj> >
 
           class T = std::projected_value_t<I, Proj> >
−
requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
+
requires std::indirect_binary_predicate
−
                                        const T*>
+
            <ranges::equal_to, std::projected<I, Proj>, const T*>
 
constexpr ranges::subrange<I>
 
constexpr ranges::subrange<I>
 
     find_last( I first, S last, const T& value, Proj proj = {} );
 
     find_last( I first, S last, const T& value, Proj proj = {} );
Line 25: Line 25:
 
           class T,
 
           class T,
 
           class Proj = std::identity >
 
           class Proj = std::identity >
−
requires std::indirect_binary_predicate<ranges::equal_to,
+
requires std::indirect_binary_predicate
−
                                        std::projected<ranges::iterator_t<R>, Proj>,
+
            <ranges::equal_to,
−
                                        const T*>
+
              std::projected<ranges::iterator_t<R>, Proj>, const T*>
 
constexpr ranges::borrowed_subrange_t<R>
 
constexpr ranges::borrowed_subrange_t<R>
 
     find_last( R&& r, const T& value, Proj proj = {} );
 
     find_last( R&& r, const T& value, Proj proj = {} );
Line 34: Line 34:
 
           class Proj = std::identity,
 
           class Proj = std::identity,
 
           class T = std::projected_value_t<iterator_t<R>, Proj> >
 
           class T = std::projected_value_t<iterator_t<R>, Proj> >
−
requires std::indirect_binary_predicate<ranges::equal_to,
+
requires std::indirect_binary_predicate
−
                                        std::projected<ranges::iterator_t<R>, Proj>,
+
            <ranges::equal_to,
−
                                        const T*>
+
              std::projected<ranges::iterator_t<R>, Proj>, const T*>
 
constexpr ranges::borrowed_subrange_t<R>
 
constexpr ranges::borrowed_subrange_t<R>
 
     find_last( R&& r, const T& value, Proj proj = {} );
 
     find_last( R&& r, const T& value, Proj proj = {} );
Line 48: Line 48:
 
}}
 
}}
 
{{dcl|since=c++23|num=4|1=
 
{{dcl|since=c++23|num=4|1=
−
template< ranges::forward_range R, class Proj = std::identity,
+
template< ranges::forward_range R,
−
           std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
+
          class Proj = std::identity,
−
              Pred >
+
           std::indirect_unary_predicate
 +
              <std::projected<ranges::iterator_t<R>, Proj>> Pred >
 
constexpr ranges::borrowed_subrange_t<R>
 
constexpr ranges::borrowed_subrange_t<R>
 
     find_last_if( R&& r, Pred pred, Proj proj = {} );
 
     find_last_if( R&& r, Pred pred, Proj proj = {} );
Line 62: Line 63:
 
}}
 
}}
 
{{dcl|since=c++23|num=6|1=
 
{{dcl|since=c++23|num=6|1=
−
template< ranges::forward_range R, class Proj = std::identity,
+
template< ranges::forward_range R,
−
           std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
+
          class Proj = std::identity,
−
              Pred >
+
           std::indirect_unary_predicate
 +
              <std::projected<ranges::iterator_t<R>, Proj>> Pred >
 
constexpr ranges::borrowed_subrange_t<R>
 
constexpr ranges::borrowed_subrange_t<R>
 
     find_last_if_not( R&& r, Pred pred, Proj proj = {} );
 
     find_last_if_not( R&& r, Pred pred, Proj proj = {} );
Line 76: Line 78:
  
 
@5@ {{tt|find_last_if_not}} searches for the last element in the range {{range|first|last}} for which predicate {{c|pred}} returns {{c|false}}.
 
@5@ {{tt|find_last_if_not}} searches for the last element in the range {{range|first|last}} for which predicate {{c|pred}} returns {{c|false}}.
 +
 
@2,4,6@ Same as {{v|1,3,5}}, but uses {{c|r}} as the source range, as if using {{c|ranges::begin(r)}} as {{c|first}} and {{c|ranges::end(r)}} as {{c|last}}.
 
@2,4,6@ Same as {{v|1,3,5}}, but uses {{c|r}} as the source range, as if using {{c|ranges::begin(r)}} as {{c|first}} and {{c|ranges::end(r)}} as {{c|last}}.
  
Line 90: Line 93:
  
 
===Return value===
 
===Return value===
−
@1,2,3@ Let {{tt|i}} be the last iterator in the range {{range|first|last}} for which {{tt|E}} is {{c|true}}. Returns {{c|ranges::subrange<I>{i, last} }}, or {{c|ranges::subrange<I>{last, last} }} if no such iterator is found.
+
@1,3,5@ Let {{c|i}} be the last iterator in the range {{range|first|last}} for which {{c|E}} is {{c|true}}.
 +
@@ Returns {{c|ranges::subrange<I>{i, last}<!---->}}, or {{c|ranges::subrange<I>{last, last}<!---->}} if no such iterator is found.
 +
:@1@ {{c|E}} is {{c|1=bool(std::invoke(proj, *i) == value)}}.
 +
:@3@ {{c|E}} is {{c|bool(std::invoke(pred, std::invoke(proj, *i)))}}.
 +
:@5@ {{c|E}} is {{c|bool(!std::invoke(pred, std::invoke(proj, *i)))}}.
  
−
@2,4,6@ Same as {{v|1,2,3}} but the return type is {{c|ranges::borrowed_subrange_t<I>}}.
+
@2,4,6@ Same as {{v|1,3,5}} but the return type is {{c|ranges::borrowed_subrange_t<I>}}.
  
 
===Complexity===
 
===Complexity===
Line 98: Line 105:
  
 
===Notes===
 
===Notes===
−
{{tt|ranges::find_last}}, {{tt|ranges::find_last_if}}, {{tt|ranges::find_last_if_not}} have better efficiency on common implementations if {{c|I}} models {{lconcept|bidirectional_iterator}} or (better) {{lconcept|random_access_iterator}}.
+
{{tt|ranges::find_last}}, {{tt|ranges::find_last_if}}, {{tt|ranges::find_last_if_not}} have better efficiency on common implementations if {{tt|I}} models {{lconcept|bidirectional_iterator}} or (better) {{lconcept|random_access_iterator}}.
  
 
{{ftm begin}}
 
{{ftm begin}}
 
{{ftm|std=C++23|value=202207L|__cpp_lib_ranges_find_last|{{tt|ranges::find_last}},<br>{{tt|ranges::find_last_if}},<br>{{tt|ranges::find_last_if_not}}}}
 
{{ftm|std=C++23|value=202207L|__cpp_lib_ranges_find_last|{{tt|ranges::find_last}},<br>{{tt|ranges::find_last_if}},<br>{{tt|ranges::find_last_if_not}}}}
−
{{ftm|__cpp_lib_algorithm_default_value_type|value=202403|std=C++26|[[cpp/language/list initialization|List-initialization]] for algorithms {{vl|1,2}}}}
+
{{ftm|__cpp_lib_algorithm_default_value_type|value=202403L|std=C++26|[[cpp/language/list initialization|List-initialization]] for algorithms {{vl|1,2}}}}
 
{{ftm end}}
 
{{ftm end}}
  
Line 108: Line 115:
 
These implementations only show the slower algorithm used when {{c|I}} models {{lconcept|forward_iterator}}.
 
These implementations only show the slower algorithm used when {{c|I}} models {{lconcept|forward_iterator}}.
 
{{eq impl
 
{{eq impl
−
|title1=find_last (1-2)|ver1=1|1=
+
|title1=find_last (1,2)|ver1=1|1=
 
struct find_last_fn
 
struct find_last_fn
 
{
 
{
Line 114: Line 121:
 
             class Proj = std::identity,
 
             class Proj = std::identity,
 
             class T = std::projected_value_t<iterator_t<R>, Proj>>
 
             class T = std::projected_value_t<iterator_t<R>, Proj>>
−
     requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
+
     requires std::indirect_binary_predicate
−
                                            const T*>
+
                <ranges::equal_to, std::projected<I, Proj>, const T*>
 
     constexpr ranges::subrange<I>
 
     constexpr ranges::subrange<I>
 
         operator()(I first, S last, const T &value, Proj proj = {}) const
 
         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.
 
         // Note: if I is mere forward_iterator, we may only go from begin to end.
−
         I found {};
+
         std::optional<I> found;
 
         for (; first != last; ++first)
 
         for (; first != last; ++first)
 
             if (std::invoke(proj, *first) == value)
 
             if (std::invoke(proj, *first) == value)
 
                 found = first;
 
                 found = first;
−
 
+
       
−
         if (found == I {})
+
         if (!found)
 
             return {first, first};
 
             return {first, first};
−
 
+
       
−
         return {found, std::ranges::next(found, last)};
+
         return {*found, std::ranges::next(*found, last)};
 
     }
 
     }
−
 
+
   
 
     template<ranges::forward_range R,
 
     template<ranges::forward_range R,
 
             class Proj = std::identity,
 
             class Proj = std::identity,
 
             class T = std::projected_value_t<iterator_t<R>, Proj>>
 
             class T = std::projected_value_t<iterator_t<R>, Proj>>
−
     requires std::indirect_binary_predicate<ranges::equal_to,
+
     requires std::indirect_binary_predicate
−
                                            std::projected<ranges::iterator_t<R>, Proj>,
+
                <ranges::equal_to,
−
                                            const T*>
+
                  std::projected<ranges::iterator_t<R>, Proj>, const T*>
 
     constexpr ranges::borrowed_subrange_t<R>
 
     constexpr ranges::borrowed_subrange_t<R>
 
         operator()(R&& r, const T &value, Proj proj = {}) const
 
         operator()(R&& r, const T &value, Proj proj = {}) const
Line 145: Line 152:
  
 
inline constexpr find_last_fn find_last;
 
inline constexpr find_last_fn find_last;
−
|title2=find_last_if (3-4)|ver2=3|2=
+
|title2=find_last_if (3,4)|ver2=3|2=
 
struct find_last_if_fn
 
struct find_last_if_fn
 
{
 
{
Line 155: Line 162:
 
     {
 
     {
 
         // Note: if I is mere forward_iterator, we may only go from begin to end.
 
         // Note: if I is mere forward_iterator, we may only go from begin to end.
−
         I found {};
+
         std::optional<I> found;
 
         for (; first != last; ++first)
 
         for (; first != last; ++first)
 
             if (std::invoke(pred, std::invoke(proj, *first)))
 
             if (std::invoke(pred, std::invoke(proj, *first)))
 
                 found = first;
 
                 found = first;
−
 
+
       
−
         if (found == I {})
+
         if (!found)
 
             return {first, first};
 
             return {first, first};
−
 
+
       
−
         return {found, std::ranges::next(found, last)};
+
         return {*found, std::ranges::next(*found, last)};
 
     }
 
     }
−
 
+
   
 
     template<ranges::forward_range R, class Proj = std::identity,
 
     template<ranges::forward_range R, class Proj = std::identity,
−
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
+
             std::indirect_unary_predicate
−
                Pred>
+
                <std::projected<ranges::iterator_t<R>, Proj>> Pred>
 
     constexpr ranges::borrowed_subrange_t<R>
 
     constexpr ranges::borrowed_subrange_t<R>
 
         operator()(R&& r, Pred pred, Proj proj = {}) const
 
         operator()(R&& r, Pred pred, Proj proj = {}) const
Line 178: Line 185:
  
 
inline constexpr find_last_if_fn find_last_if;
 
inline constexpr find_last_if_fn find_last_if;
−
|title3=find_last_if_not (5-6)|ver3=5|3=
+
|title3=find_last_if_not (5,6)|ver3=5|3=
 
struct find_last_if_not_fn
 
struct find_last_if_not_fn
 
{
 
{
Line 188: Line 195:
 
     {
 
     {
 
         // Note: if I is mere forward_iterator, we may only go from begin to end.
 
         // Note: if I is mere forward_iterator, we may only go from begin to end.
−
         I found {};
+
         std::optional<I> found;
 
         for (; first != last; ++first)
 
         for (; first != last; ++first)
 
             if (!std::invoke(pred, std::invoke(proj, *first)))
 
             if (!std::invoke(pred, std::invoke(proj, *first)))
 
                 found = first;
 
                 found = first;
−
 
+
       
−
         if (found == I {})
+
         if (!found)
 
             return {first, first};
 
             return {first, first};
−
 
+
       
−
         return {found, std::ranges::next(found, last)};
+
         return {*found, std::ranges::next(*found, last)};
 
     }
 
     }
−
 
+
   
 
     template<ranges::forward_range R, class Proj = std::identity,
 
     template<ranges::forward_range R, class Proj = std::identity,
−
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
+
             std::indirect_unary_predicate
−
                Pred>
+
                <std::projected<ranges::iterator_t<R>, Proj>> Pred>
 
     constexpr ranges::borrowed_subrange_t<R>
 
     constexpr ranges::borrowed_subrange_t<R>
 
         operator()(R&& r, Pred pred, Proj proj = {}) const
 
         operator()(R&& r, Pred pred, Proj proj = {}) const
Line 226: Line 233:
 
{
 
{
 
     namespace ranges = std::ranges;
 
     namespace ranges = std::ranges;
−
 
+
   
 
     constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};
 
     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 i1 = ranges::find_last(v.begin(), v.end(), 3);
Line 241: Line 248:
 
         static_assert(i2.begin() == v.end());
 
         static_assert(i2.begin() == v.end());
 
     }
 
     }
−
 
+
   
 
     auto abs = [](int x) { return x < 0 ? -x : x; };
 
     auto abs = [](int x) { return x < 0 ? -x : x; };
−
 
+
   
 
     {
 
     {
 
         auto pred = [](int x) { return x == 3; };
 
         auto pred = [](int x) { return x == 3; };
Line 258: Line 265:
 
         static_assert(i2.begin() == v.end());
 
         static_assert(i2.begin() == v.end());
 
     }
 
     }
−
 
+
   
 
     {
 
     {
 
         auto pred = [](int x) { return x == 1 or x == 2; };
 
         auto pred = [](int x) { return x == 1 or x == 2; };
Line 273: Line 280:
 
         static_assert(i2.begin() == v.end());
 
         static_assert(i2.begin() == v.end());
 
     }
 
     }
−
 
+
   
 
     using P = std::pair<std::string_view, int>;
 
     using P = std::pair<std::string_view, int>;
 
     std::forward_list<P> list
 
     std::forward_list<P> list
Line 281: Line 288:
 
     };
 
     };
 
     auto cmp_one = [](const std::string_view &s) { return s == "one"; };
 
     auto cmp_one = [](const std::string_view &s) { return s == "one"; };
−
 
+
   
 
     // find latest element that satisfy the comparator, and projecting pair::first
 
     // find latest element that satisfy the comparator, and projecting pair::first
 
     const auto subrange = ranges::find_last_if(list, cmp_one, &P::first);
 
     const auto subrange = ranges::find_last_if(list, cmp_one, &P::first);
−
 
+
   
−
     // print the found element and the "tail" after it
+
     std::cout << "The found element and the tail after it are:\n";
 
     for (P const& e : subrange)
 
     for (P const& e : subrange)
 
         std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
 
         std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
 
     std::cout << '\n';
 
     std::cout << '\n';
−
 
+
   
 
#if __cpp_lib_algorithm_default_value_type
 
#if __cpp_lib_algorithm_default_value_type
 
     const auto i3 = ranges::find_last(list, {"three", 3}); // (2) C++26
 
     const auto i3 = ranges::find_last(list, {"three", 3}); // (2) C++26
Line 298: Line 305:
 
}
 
}
 
|output=
 
|output=
 +
The found element and the tail after it are:
 
{"one", 4} {"two", 5} {"three", 6}
 
{"one", 4} {"two", 5} {"three", 6}
 
}}
 
}}

Latest revision as of 18:41, 16 July 2024

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
       
       
    
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
       
       
Permutation operations
Fold operations
Numeric operations
(C++23)            
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
(1)
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 = {} );
 (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)
(2)
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 = {} );
 (since C++23)
(until C++26)
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>

    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)

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.

Contents

[edit] 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

[edit] Return value

1,3,5) 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.
1) E is bool(std::invoke(proj, *i) == value).
3) E is bool(std::invoke(pred, std::invoke(proj, *i))).
5) E is bool(!std::invoke(pred, std::invoke(proj, *i))).
2,4,6) Same as (1,3,5) but the return type is ranges::borrowed_subrange_t<I>.

[edit] Complexity

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

[edit] 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)

[edit] 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 Proj = std::identity,
             class T = std::projected_value_t<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, std::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->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.
        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, 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.
        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, 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;

[edit] Example

Run this code
#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}

[edit] See also

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