std::partition

 

C++

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Freestanding and hosted

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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 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

Return value

Iterator to the first element of the second group.

Complexity

Exactly last-first applications of the predicate and at most last-first swaps. If ForwardIt meets the requirements of Template:concept at most (last-first)/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 1

Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <iterator>
#include <vector>
 
bool is_even(int i) { return i % 2 == 0; }
 
int main()
{
    std::vector<int> v;
    for (int i = 0; i < 10; ++i) v.push_back(i);
 
    std::cout << "Original vector:\n    ";
    std::copy(std::begin (v), std::end (v), std::ostream_iterator<int>(std::cout, " "));
 
    // Partition the vector
    auto it = std::partition(v.begin(), v.end(), std::ptr_fun(is_even));
 
    std::cout << "\nPartitioned vector:\n    ";
    std::copy(std::begin (v), std::end (v), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\nBefore partition:\n    ";
    std::copy(std::begin (v), it, std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\nAfter partition:\n    ";
    std::copy(it, std::end (v), std::ostream_iterator<int>(std::cout, " "));
}

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 
Before partition:
    0 8 2 6 4 
After partition:
    5 3 7 1 9

Example 2

Run this code

#include <iterator>
#include <algorithm>
#include <forward_list>
#include <iostream>
 
template <class ForwardIt>
 ForwardIt choose_pivot(ForwardIt first, ForwardIt last)
 {
   return std::next(first, std::distance(first,last)/2);
 }
 
template <class ForwardIt>
 void quicksort(ForwardIt first, ForwardIt last)
 {
    if(first == last) return;
 
    auto pivot = *choose_pivot(first,last);
 
    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::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
 
   quicksort(std::begin(fl), std::end(fl));
 
   for(int fi : fl) std::cout << fi << ' ';
   std::cout << '\n';
}

Output:

-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)