std::partial_sort_copy

 

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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 InputIt, class RandomIt > RandomIt partial_sort_copy( InputIt first, InputIt last,                             RandomIt d_first, RandomIt d_last );	(1)	(constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt, class RandomIt > RandomIt partial_sort_copy( ExecutionPolicy&& policy,                             ForwardIt first, ForwardIt last,                             RandomIt d_first, RandomIt d_last );	(2)	(since C++17)
template< class InputIt, class RandomIt, class Compare > RandomIt partial_sort_copy( InputIt first, InputIt last,                             RandomIt d_first, RandomIt d_last,                             Compare comp );	(3)	(constexpr since C++20)
template< class ExecutionPolicy,           class ForwardIt, class RandomIt, class Compare > RandomIt partial_sort_copy( ExecutionPolicy&& policy,                             ForwardIt first, ForwardIt last,                             RandomIt d_first, RandomIt d_last,                             Compare comp );	(4)	(since C++17)

Sorts some of the elements in the range [first, last) in ascending order, storing the result in the range [d_first, d_last).

At most d_last - d_first of the elements are placed sorted to the range [d_first, d_first + n). n is the number of elements to sort (n = min(last - first, d_last - d_first)). The order of equal elements is not guaranteed to be preserved.

1) Elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).

3) Elements are sorted with respect to comp.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if

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 *first is not writable to d_first, the program is ill-formed.

If any of the following conditions is satisfied, the behavior is undefined:

The type of *d_first is not Swappable.	(until C++11)
RandomIt is not ValueSwappable. The type of *d_first is not MoveConstructible. The type of *d_first is not MoveAssignable.	(since C++11)

Parameters

first, last	-	the range of elements to sort
d_first, d_last	-	random access iterators defining the destination range
policy	-	the execution policy to use. See execution policy for details.
comp	-	comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1& a, const Type2& b); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- InputIt must meet the requirements of LegacyInputIterator.
- ForwardIt must meet the requirements of LegacyForwardIterator.
- RandomIt must meet the requirements of LegacyRandomAccessIterator.
- Compare must meet the requirements of Compare.

Return value

An iterator to the element defining the upper boundary of the sorted range, i.e. d_first + std::min(last - first, d_last - d_first).

Complexity

Given \(\scriptsize N\)N as last - first, \(\scriptsize D\)D as d_last - d_first:

1,2) Approximately \(\scriptsize N \cdot \log(\min(N,D))\)N·log(min(N,D)) comparisons using operator<(until C++20)std::less{}(since C++20).

3,4) Approximately \(\scriptsize N \cdot \log(\min(N,D))\)N·log(min(N,D)) applications of the comparator comp.

Exceptions

The overloads with a template parameter named ExecutionPolicy report 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

See also the implementations in libstdc++ and libc++.

Example

The following code sorts a vector of integers and copies them into a smaller and a larger vector.

Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <type_traits>
#include <vector>
 
void println(std::string_view rem, const auto& v)
{
    std::cout << rem;
    if constexpr (std::is_scalar_v<std::decay_t<decltype(v)>>)
        std::cout << v;
    else
        for (int e : v)
            std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    const auto v0 = {4, 2, 5, 1, 3};
    std::vector<int> v1{10, 11, 12};
    std::vector<int> v2{10, 11, 12, 13, 14, 15, 16};
    std::vector<int>::iterator it;
 
    it = std::partial_sort_copy(v0.begin(), v0.end(), v1.begin(), v1.end());
    println("Writing to the smaller vector in ascending order gives: ", v1);
 
    if (it == v1.end())
        println("The return value is the end iterator", ' ');
 
    it = std::partial_sort_copy(v0.begin(), v0.end(), v2.begin(), v2.end(),
                                std::greater<int>());
 
    println("Writing to the larger vector in descending order gives: ", v2);
    println("The return value is the iterator to ", *it);
}

Output:

Writing to the smaller vector in ascending order gives: 1 2 3
The return value is the end iterator
Writing to the larger vector in descending order gives: 5 4 3 2 1 15 16
The return value is the iterator to 15

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
P0896R4	C++98	*first was not required to be writable to d_first	the program is ill-formed if not writable

See also

partial_sort	sorts the first N elements of a range (function template)
sort	sorts a range into ascending order (function template)
stable_sort	sorts a range of elements while preserving order between equal elements (function template)
ranges::partial_sort_copy (C++20)	copies and partially sorts a range of elements (niebloid)