std::partition_point() algorithm
- od C++20
- od C++11
template< class ForwardIt, class UnaryPredicate >
constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );
// (1)
template< class ForwardIt, class UnaryPredicate >
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );
Examines the partitioned (as if by std::partition) 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
.
Parameters
first last | The partitioned range of elements to examine |
policy | The execution policy to use. See execution policy for details. |
p | Unary predicate which returns The expression |
Type requirements
ForwardIt | LegacyForwardIterator |
UnaryPredicate | Predicate |
Return value
The iterator past the end of the first partition within [first
; last
) or last if all elements satisfy p
.
Complexity
Given N
as std::distance(first, last)
:
Performs O(log(N)) applications of the predicate p
.
However, for non-LegacyRandomAccessIterators, the number of iterator increments is O(N).
Exceptions
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
- If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicy
is one of the standard policies,std::terminate
is called. For any otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Possible implementation
partition_point (1)
template<class ForwardIt, class UnaryPredicate>
constexpr //< since C++20
ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPredicate 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;
}
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 [&](auto const& e) { return e < value; });
.
Examples
#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());
}
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
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