std::partition
Min standard notice:
Header: <algorithm>
- Reorders the elements in the range [first,last) in such a way that all elements for which the predicate p returns true precede all elements for which predicate p returns false. Relative order of the elements is not preserved.
# Declarations
template< class ForwardIt, class UnaryPred >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );
(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
ForwardIt partition( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, UnaryPred p );
(since C++17)
# Parameters
first, last: the range of elements to reorderpolicy: the execution policy to usep: unary predicate which returns true if the element should be ordered before other elements. 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).
# Return value
Iterator to the first element of the second group.
# Example
#include <algorithm>
#include <forward_list>
#include <iostream>
#include <iterator>
#include <vector>
template<class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
{
if (first == last)
return;
auto pivot = *std::next(first, std::distance(first, last) / 2);
auto middle1 = std::partition(first, last, [pivot](const auto& em)
{
return em < pivot;
});
auto middle2 = std::partition(middle1, last, [pivot](const auto& em)
{
return !(pivot < em);
});
quicksort(first, middle1);
quicksort(middle2, last);
}
int main()
{
std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
std::cout << "Original vector: ";
for (int elem : v)
std::cout << elem << ' ';
auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
std::cout << "\nPartitioned vector: ";
std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
std::cout << "* ";
std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
std::cout << "\nUnsorted list: ";
for (int n : fl)
std::cout << n << ' ';
quicksort(std::begin(fl), std::end(fl));
std::cout << "\nSorted using quicksort: ";
for (int fi : fl)
std::cout << fi << ' ';
std::cout << '\n';
}
# Defect reports
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 498 | C++98 | std::partition required first andlast to be LegacyBidirectionalIterator | only required to beLegacyForwardIterator |
| LWG 2150 | C++98 | std::partition was only required to place one elementsatisfying p before one element not satisfying p | corrected therequirement |