std::ranges::partition_point
| Defined in header <algorithm>
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| Call signature |
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template< std::forward_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr I
partition_point( I first, S last, Pred pred, Proj proj = {} );
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(1) | (since C++20) |
template< ranges::forward_range R,
class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>
partition_point( R&& r, Pred pred, Proj proj = {} );
|
(2) | (since C++20) |
Returns the iterator iter indicating the partition point of the source range [first, last) or r: all elements (projected by proj) before iter satisfy pred, while all elements starting from iter do not.
If the elements e of the source range are not partitioned with respect to the expression bool(std::invoke(pred, std::invoke(proj, e))), the behavior is undefined.
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 source range |
| r | - | the source range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
Return value
As described above.
Complexity
Given N as ranges::distance(first, last) or ranges::distance(r):
pred and proj.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); });.
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
See also
(C++11) |
locates the partition point of a partitioned range (function template) |
(C++20) |
checks whether a range is sorted (algorithm function object) |
(C++20) |
finds the first element not less than the given value using binary search (algorithm function object) |