std::partition - cppreference.com (original) (raw)

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 all following conditions are satisfied:

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

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Exceptions
• 5 Possible implementation
• 6 Example
• 7 Defect reports
• 8 See also

[edit] Parameters

[edit] Return value

Iterator to the first element of the second group.

[edit] 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.

1. \(\scriptsize O(N)\)O(N) applications of p.

\(\scriptsize O(N \cdot log(N))\)O(N·log(N)) swaps.

[edit] 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.

[edit] 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; }

[edit] Example

#include #include #include #include #include   template 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 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(std::cout, " ")); std::cout << "* "; std::copy(it, std::end(v), std::ostream_iterator(std::cout, " "));   std::forward_list 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

[edit] Defect reports

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

[edit] See also

| | determines if the range is partitioned by the given predicate (function template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | divides elements into two groups while preserving their relative order (function template) [edit] | | | divides a range of elements into two groups(algorithm function object)[edit] |