std::partial_sort_copy

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partial_sort_copy
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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 (std::min(std::distance(first, last), 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:

(until C++11)
(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(std::distance(first, last), d_last - d_first).

Complexity

Given N as std::distance(first, last), D as d_last - d_first:

1,2) Approximately N·log(min(N,D)) comparisons using operator<(until C++20)std::less{}(since C++20).
3,4) Approximately 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

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