std::partition_point

 

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Defined in header <algorithm>
template<class ForwardIt, class UnaryPred > ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPred p );		(since C++11) (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.

Parameters

first, last	-	the pair of iterators defining the partitioned range of elements to examine
p	-	unary predicate which returns ​true for the elements found in the beginning of the range. 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). ​
Type requirements
- ForwardIt must meet the requirements of LegacyForwardIterator.
- UnaryPred must meet the requirements of Predicate.

Return value

The iterator past the end of the first partition within [first, last) or last if all elements satisfy p.

Complexity

Given \(\scriptsize N\)N as std::distance(first, last), performs \(\scriptsize O(log(N))\)O(log(N)) applications of the predicate p.

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 [&](constauto& e){return e < value;});.

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;}

Example

Run this code

#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());   constauto pp = std::partition_point(v.cbegin(), v.cend(), is_even);constauto 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

See also

findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template)
is_sorted (C++11)	checks whether a range is sorted into ascending order (function template)
lower_bound	returns an iterator to the first element not less than the given value (function template)
ranges::partition_point (C++20)	locates the partition point of a partitioned range (algorithm function object)