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

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< cpp‎ | algorithm‎ | ranges
 
 
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)
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
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations Permutation operations


C library
Numeric operations
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
find_lastfind_last_iffind_last_if_not
(C++23)(C++23)(C++23)  

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
(C++23)
(C++23)  
(C++23)
(C++23)  
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 algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

first, last - the iterator-sentinel pair defining 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

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]
returns true if one sequence is a subsequence of another
(algorithm function object)[edit]
determines if an element exists in a partially-ordered range
(algorithm function object)[edit]
checks if the range contains the given element or subrange
(algorithm function object)[edit]

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