std::ranges::partition_point
std::ranges
<algorithm>
class Proj = std::identity,
std::indirect_unary_predicate <std::projected <I, Proj>> Pred >
constexpr I
class Proj = std::identity,
std::indirect_unary_predicate <
std::projected <ranges::iterator_t <R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t <R>
Examines the partitioned (as if by ranges::partition ) range [first, last) or r and locates the end of the first partition, that is, the projected element that does not satisfy pred or last if all projected elements satisfy pred.
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.
[edit] Parameters
[edit] Return value
The iterator past the end of the first partition within [first, last) or the iterator equal to last if all projected elements satisfy pred.
[edit] Complexity
Given N = ranges::distance (first, last), performs O(log N) applications of the predicate pred and projection proj.
However, if sentinels don't model std::sized_sentinel_for <I>, the number of iterator increments is O(N).
[edit] Notes
This algorithm is a more general form of ranges::lower_bound, which can be expressed in terms of ranges::partition_point with the predicate [&](auto const& e) { return std::invoke (pred, e, value); });.
[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::ranges::partition (v, is_even); print_seq("After partitioning, v: ", v.cbegin(), v.cend()); const auto pp = std::ranges::partition_point(v, is_even); const auto i = std::ranges::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: 2 4 6 8 5 3 7 1 9 Partition point is at 4; v[4] = 5 First partition (all even elements): 2 4 6 8 Second partition (all odd elements): 5 3 7 1 9
[edit] See also
(algorithm function object)[edit]