std::partition

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

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 ForwardIt, class UnaryPred > ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );	(1)	(constexpr since C++20)
template<class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt partition( ExecutionPolicy&& policy,                      ForwardIt first, ForwardIt last, UnaryPred p );	(2)	(since C++17)

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:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

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

Parameters

first, last	-	the pair of iterators defining the range of elements to reorder
policy	-	the execution policy to use
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
- ForwardIt must meet the requirements of LegacyForwardIterator.
- UnaryPred must meet the requirements of Predicate.

Return value

Iterator to the first element of the second group.

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.

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

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

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

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;}

Example

Run this code

#include <algorithm>#include <forward_list>#include <iostream>#include <iterator>#include <vector>   template<class ForwardIt>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](constauto& em){return em < pivot;});auto middle2 = std::partition(middle1, last, [pivot](constauto& 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: ";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<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: ";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

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 498	C++98	std::partition required first and last to be LegacyBidirectionalIterator	only required to be LegacyForwardIterator
LWG 2150	C++98	std::partition was only required to place one element satisfying p before one element not satisfying p	corrected the requirement

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)
ranges::partition (C++20)	divides a range of elements into two groups (algorithm function object)