std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not
Defined in header <algorithm>
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Call signature |
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(1) | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T, |
(since C++23) (until C++26) |
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template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(since C++26) | |
(2) | ||
template< ranges::forward_range R, class T, |
(since C++23) (until C++26) |
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template< ranges::forward_range R, class Proj = std::identity, |
(since C++26) | |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(3) | (since C++23) |
template< ranges::forward_range R, class Proj = std::identity, |
(4) | (since C++23) |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(5) | (since C++23) |
template< ranges::forward_range R, class Proj = std::identity, |
(6) | (since C++23) |
Returns the last element in the range [
first,
last)
that satisfies specific criteria:
find_last
searches for an element equal to value.find_last_if
searches for the last element in the range [
first,
last)
for which predicate pred returns true.find_last_if_not
searches for the last element in the range [
first,
last)
for which predicate pred returns false.The function-like entities described on this page are algorithm function objects (informally known as 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.
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
[
first,
last)
for which E is true.[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
#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 (algorithm function object) |
(C++20)(C++20)(C++20) |
finds the first element satisfying specific criteria (algorithm function object) |
(C++20) |
searches for the first occurrence of a range of elements (algorithm function object) |
(C++20) |
returns true if one sequence is a subsequence of another (algorithm function object) |
(C++20) |
determines if an element exists in a partially-ordered range (algorithm function object) |
(C++23)(C++23) |
checks if the range contains the given element or subrange (algorithm function object) |