std::partition

Defined in header <algorithm>

template< class BidirIt, class UnaryPredicate >
BidirIt partition( BidirIt first, BidirIt last, UnaryPredicate p );

(until C++11)

template< class ForwardIt, class UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );

(since C++11)

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.

Parameters

first, last	-	the range of elements to reorder
p	-	unary predicate which returns `true` if the element should be ordered before other elements. The signature of the predicate function should be equivalent to the following: `bool pred(const Type &a);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The type `Type` must be such that an object of type `ForwardIt` can be dereferenced and then implicitly converted to `Type`.
Type requirements
- `BidirIt` must meet the requirements of `BidirectionalIterator`.
- `ForwardIt` must meet the requirements of `ValueSwappable` and `ForwardIterator`. However, the operation is more efficient if `ForwardIt` also satisfies the requirements of `BidirectionalIterator`
- `UnaryPredicate` must meet the requirements of `Predicate`.

Return value

Iterator to the first element of the second group.

Complexity

Exactly std::distance(first,last) applications of the predicate and at most std::distance(first,last) swaps. If ForwardIt meets the requirements of BidirectionalIterator at most std::distance(first,last)/2 swaps are done.

Possible implementation

template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
    if (first == last) return first;
    ForwardIt part(first++);
    if (first == last) return p(*part) ? first : part;
    while (first != last) {
        if (p(*part))
            ++part;
        else if (p(*first)) {
            iter_swap(part, first);
            ++part;
        } ++first;
    } return part;
}

Example

#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

is_partitioned (C++11)	determines if the range is partitioned by the given predicate (function template)
stable_partition	divides elements into two groups while preserving their relative order (function template)
std::experimental::parallel::partition (parallelism TS)	parallelized version of `std::partition` (function template)

Links:

http://en.cppreference.com/w/cpp/algorithm/partition

doc_CPP

2025-01-10 15:47:30

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