std::partition (3) - Linux Manuals

std::partition: std::partition

NAME

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