std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not
来自cppreference.com
| 在标头 <algorithm> 定义
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| 调用签名 |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > |
(1) | (C++23 起) |
| template< ranges::forward_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, |
(2) | (C++23 起) |
| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(3) | (C++23 起) |
| template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> |
(4) | (C++23 起) |
| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(5) | (C++23 起) |
| template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> |
(6) | (C++23 起) |
返回范围 [first, last) 中符合特定条件的最后一个元素：
1)
find_last 搜索等于 value 的元素。3)
find_last_if 搜索范围 [first, last) 中谓词 pred 返回 true 的最后一个元素。5)
find_last_if_not 搜索范围 [first, last) 中谓词 pred 返回 false 的最后一个元素。此页面上描述的仿函数实体是 niebloid，即：
实际上，它们能以函数对象，或者某些特殊编译器扩展实现。
参数
| first, last | - | 要检查的元素的范围 |
| r | - | 要检查的元素的范围 |
| value | - | 和元素进行比较的值 |
| pred | - | 应用到投影后元素的谓词 |
| proj | - | 应用到元素的投影 |
返回值
1,2,3) 让
i 是 [first, last) 范围内的最后一个迭代器，其中 E 是 true。返回 ranges::subrange<I>{i, last}，或 ranges::subrange<I>{last, last} 如果不存在这样的迭代器。复杂度
最多应用 last - first 次谓词和投影。
注解
如果ranges::find_last、ranges::find_last_if、ranges::find_last_if_not 的 I 为 bidirectional_iterator 或（更好的）random_access_iterator，则它们在通用实现上的效率更高。
| 功能特性测试宏 | 值 | 标准 | 备注 |
|---|---|---|---|
__cpp_lib_ranges_find_last |
202207L | (C++23) | ranges::find_last, ranges::find_last_if, ranges::find_last_if_not
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可能的实现
这个实现只展示 I 为 forward_iterator 时较慢的算法。
| find_last (1-2) |
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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; |
示例
运行此代码
#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'; }
输出：
{"one", 4} {"two", 5} {"three", 6}参阅
| (C++20) |
查找特定范围中最后出现的元素序列 (niebloid) |
| (C++20)(C++20)(C++20) |
查找满足特定条件的的第一个元素 (niebloid) |
| (C++20) |
搜索一个元素范围 (niebloid) |
| (C++20) |
若一个序列是另一个的子列则返回 true (niebloid) |
| (C++20) |
确定元素是否存在于某范围中 (niebloid) |
| (C++23)(C++23) |
检查范围是否包含给定的元素或子范围 (niebloid) |