NAME¶

std::partition - std::partition

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class UnaryPred >
ForwardIt partition( ForwardIt first, ForwardIt last, (1) (constexpr since C++20)
UnaryPred p );
template< class ExecutionPolicy, class ForwardIt, class
UnaryPred >

ForwardIt partition( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, ForwardIt last,
UnaryPred p );

1) 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.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

If
the type of *first is not Swappable
(until C++11)
ForwardIt is not ValueSwappable
(since C++11), the behavior is undefined.

Parameters¶

first, last - the range of elements to reorder
policy - the execution policy to use. See execution policy for details.
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
p - 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¶

Iterator to the first element of the second group.

Complexity¶

Given \(\scriptsize N\)N as std::distance(first, last):

1) Exactly \(\scriptsize N\)N applications of p.
At most \(\scriptsize N/2\)N/2 swaps if ForwardIt meets the requirements of
LegacyBidirectionalIterator, and at most \(\scriptsize N\)N swaps otherwise.
2) \(\scriptsize O(N)\)O(N) applications of p.
\(\scriptsize O(N \cdot log(N))\)O(N·log(N)) swaps.

Exceptions¶

The overload with a template parameter named ExecutionPolicy reports 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 other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

Implements overload (1) preserving C++11 compatibility.

template<class ForwardIt, class UnaryPred>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred p)
{
first = std::find_if_not(first, last, p);
if (first == last)
return first;

for (auto i = std::next(first); i != last; ++i)
if (p(*i))
{
std::iter_swap(i, first);
++first;
}

return first;
}

Example¶

// Run this code

#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';
}

Possible output:¶

Original vector: 0 1 2 3 4 5 6 7 8 9
Partitioned vector: 0 8 2 6 4 * 5 3 7 1 9
Unsorted list: 1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
Sorted using quicksort: -8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 498 C++98 std::partition required first and only required to be
last to be LegacyBidirectionalIterator LegacyForwardIterator
std::partition was only required to place
LWG 2150 C++98 one element corrected the
satisfying p before one element not requirement
satisfying p

See also¶

is_partitioned determines if the range is partitioned by the given predicate
(C++11) (function template)
divides elements into two groups while preserving their relative
stable_partition order
(function template)
ranges::partition divides a range of elements into two groups
(C++20) (niebloid)