std::ranges:: partition_point

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.

Parameters

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 .

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) .

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());
}

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