std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not
来自cppreference.com
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| 在标头 <algorithm> 定义
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| 调用签名 |
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| (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 = {} ); |
(C++23 起) (C++26 前) |
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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 = {} ); |
(C++26 起) | |
| (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 = {} ); |
(C++23 起) (C++26 前) |
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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 = {} ); |
(C++26 起) | |
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) | (C++23 起) |
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) | (C++23 起) |
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) | (C++23 起) |
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) | (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 为之返回 true 的最后一个元素。5)
find_last_if_not 搜索范围 [first, last) 中谓词 pred 为之返回 false 的最后一个元素。2,4,6) 与 (1,3,5) 相同，但使用
r 作为源范围，就像使用 ranges::begin(r) 作为 first 和 ranges::end(r) 作为 last 一样。此页面上描述的函数式实体是算法函数对象（非正式地称为 niebloid），即：
参数
| first, last | - | 要检验的元素范围的迭代器-哨位对 |
| 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) 设
i 是 [first, last) 范围内使得 E 是 true 的最后一个迭代器。 返回
ranges::subrange<I>{i, last}，如果不存在这样的迭代器，则返回 ranges::subrange<I>{last, last}。1)
E 是 bool(std::invoke(proj, *i) == value)。3)
E 是 bool(std::invoke(pred, std::invoke(proj, *i)))。5)
E 是 bool(!std::invoke(pred, std::invoke(proj, *i)))。2,4,6) 和 (1,3,5) 类似，但是返回值类型是
ranges::borrowed_subrange_t<I>。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 | 功能特性 |
|---|---|---|---|
__cpp_lib_ranges_find_last |
202207L |
(C++23) | ranges::find_last，ranges::find_last_if，ranges::find_last_if_not
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__cpp_lib_algorithm_default_value_type |
202403L |
(C++26) | 算法的列表初始化 (1,2) |
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) |
|---|
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
{
// 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。
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;
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| 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
{
// 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。
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
{
// 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。
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;
|
示例
运行此代码
#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"; };
// 寻找满足比较器的最后一个元素，其按 pair::first 投影
const auto subrange = ranges::find_last_if(list, cmp_one, &P::first);
std::cout << "所找到的元素和其后的尾部为:\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);
}
输出：
所找到的元素和其后的尾部为:
{"one", 4} {"two", 5} {"three", 6}
参阅
(C++20) |
查找元素序列在特定范围中最后一次出现 (算法函数对象) |
(C++20)(C++20)(C++20) |
查找首个满足特定条件的元素 (算法函数对象) |
(C++20) |
搜索元素范围的首次出现 (算法函数对象) |
(C++20) |
当一个序列是另一个的子序列时返回 true (算法函数对象) |
(C++20) |
判断元素是否在偏序范围中 (算法函数对象) |
(C++23)(C++23) |
检查范围是否包含给定元素或子范围 (算法函数对象) |