std::partition

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Algorithm library

Functions
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each countcount_if mismatch equal	findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n removeremove_if remove_copyremove_copy_if	replacereplace_if replace_copyreplace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap (C++11) is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

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 BidirIt, class UnaryPredicate> BidirIt partition(BidirIt first, BidirIt last, UnaryPredicate p) { while (1) { while ((first != last) && p(*first)) { ++first; } if (first == last--) break; while ((first != last) && !p(*last)) { --last; } if (first == last) break; std::iter_swap(first++, last); } 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

See also

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)