std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not
From cppreference.com
| 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:
- 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.
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 |
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>.Complexity
At most last - first applications of the predicate and projection.
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) |
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;
|
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}
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) |