std::partition_point
<algorithm>
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPred p );
(constexpr since C++20)
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 respect to the expression bool(p(elem)), the behavior is undefined.
Contents
[edit] Parameters
The expression p(v) must be convertible to bool for every argument v 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).
ForwardIt must meet the requirements of LegacyForwardIterator.
UnaryPred must meet the requirements of Predicate.
[edit] Return value
The iterator past the end of the first partition within [first, last) or last if all elements satisfy p.
[edit] Complexity
Given \(\scriptsize N\)N as std::distance (first, last), performs \(\scriptsize O(log(N))\)O(log(N)) applications of the predicate p.
[edit] 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; });.
[edit] 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; }
[edit] Example
#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