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std::partition_point(3) C++ Standard Libary std::partition_point(3) NAME std::partition_point - std::partition_point Synopsis Defined in header <algorithm> template< class ForwardIt, class UnaryPred > (since C++11) ForwardIt partition_point( ForwardIt first, ForwardIt last, (const- expr since C++20) UnaryPred p ); Examines the partitioned range [first, last) and locates the end of the first partition, that is, the first element that does not satisfy p or last if all elements satisfy p. If the elements elem of [first, last) are not partitioned with re- spect to the expression bool(p(elem)), the behavior is undefined. Parameters first, last - the partitioned range of elements to examine unary predicate which returns true for the elements found in the beginning of the range. The expression p(v) must be convertible to bool for every argument v p - of type (possibly const) VT, where VT is the value type of ForwardIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed , nor is VT unless for VT a move is equivalent to a copy (since C++11). Type requirements - ForwardIt must meet the requirements of LegacyForwardIterator. - UnaryPred must meet the requirements of Predicate. Return value The iterator past the end of the first partition within [first, last) or last if all elements satisfy p. Complexity Given \(\scriptsize N\)N as std::distance(first, last), performs \(\scriptsize O(log(N))\)O(log(N)) applications of the predicate p. Notes This algorithm is a more general form of std::lower_bound, which can be expressed in terms of std::partition_point with the predicate [&](const auto& e) { return e < value; });. Possible implementation template<class ForwardIt, class UnaryPred> constexpr //< since C++20 ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPred p) { for (auto length = std::distance(first, last); 0 < length; ) { auto half = length / 2; auto middle = std::next(first, half); if (p(*middle)) { first = std::next(middle); length -= (half + 1); } else length = half; } return first; } Example // Run this code #include <algorithm> #include <array> #include <iostream> #include <iterator> auto print_seq = [](auto rem, auto first, auto last) { for (std::cout << rem; first != last; std::cout << *first++ << ' ') {} std::cout << '\n'; }; int main() { std::array v{1, 2, 3, 4, 5, 6, 7, 8, 9}; auto is_even = [](int i) { return i % 2 == 0; }; std::partition(v.begin(), v.end(), is_even); print_seq("After partitioning, v: ", v.cbegin(), v.cend()); const auto pp = std::partition_point(v.cbegin(), v.cend(), is_even); const auto i = std::distance(v.cbegin(), pp); std::cout << "Partition point is at " << i << "; v[" << i << "] = " << *pp << '\n'; print_seq("First partition (all even elements): ", v.cbegin(), pp); print_seq("Second partition (all odd elements): ", pp, v.cend()); } Possible output: After partitioning, v: 8 2 6 4 5 3 7 1 9 Partition point is at 4; v[4] = 5 First partition (all even elements): 8 2 6 4 Second partition (all odd elements): 5 3 7 1 9 See also find find_if finds the first element satisfying specific criteria find_if_not (function template) (C++11) is_sorted checks whether a range is sorted into as- cending order (C++11) (function template) returns an iterator to the first element not less than the lower_bound given value (function template) ranges::partition_point locates the partition point of a partitioned range (C++20) (niebloid) http://cppreference.com 2024.06.10 std::partition_point(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Notes | Possible implementation | Example | Possible output: | See also
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