Std::partition (3) Linux Manual Page

std::partition – std::partition

Synopsis

Defined in header <algorithm>

template< class BidirIt, class UnaryPredicate >                                             (until C++11)

BidirIt partition( BidirIt first, BidirIt last, UnaryPredicate p );

template< class ForwardIt, class UnaryPredicate >                                           (since C++11)

ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );                   (until C++20)

template< class ForwardIt, class UnaryPredicate >                                   (1)     (since C++20)

constexpr ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );

template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >

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

ForwardIt first, ForwardIt last, UnaryPredicate 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 the 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 does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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.
p – 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

–
BidirIt must meet the requirements of LegacyBidirectionalIterator.
–
ForwardIt must meet the requirements of ValueSwappable and LegacyForwardIterator. However, the operation is more efficient if ForwardIt also satisfies the requirements of LegacyBidirectionalIterator
–
UnaryPredicate must meet the requirements of Predicate.

Return value

Iterator to the first element of the second group.

Complexity

Given N = std::distance(first,last),
1) Exactly N applications of the predicate. At most N/2 swaps if ForwardIt meets the requirements of LegacyBidirectionalIterator, and at most N swaps otherwise.
2) O(N log N) swaps and O(N) applications of the predicate.

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

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

      for (ForwardIt 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 <iostream>

#include <iterator>

#include <vector>

#include <forward_list>
template <class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return;
    auto pivot = *std::next(first, std::distance(first, last) / 2);
    ForwardIt middle1 = std::partition(first, last,
                                       [pivot](const auto &em) { return em < pivot; });
    ForwardIt 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:\n    ";
    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:\n    ";
    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:\n    ";
    for (int n : fl)
        std::cout << n << ' ';
    std::cout << '\n';
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "Sorted using quicksort:\n    ";
    for (int fi : fl)
        std::cout << fi << ' ';
    std::cout << '\n';
}

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

std::partition (3) Linux Manual Page