Difference between revisions of "cpp/algorithm/sort"

Latest revision as of 22:49, 1 April 2024

Defined in header `<algorithm>`
template< class RandomIt > void sort( RandomIt first, RandomIt last );	(1)	(constexpr since C++20)
template< class ExecutionPolicy, class RandomIt > void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );	(2)	(since C++17)
template< class RandomIt, class Compare > void sort( RandomIt first, RandomIt last, Compare comp );	(3)	(constexpr since C++20)
template< class ExecutionPolicy, class RandomIt, class Compare > void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );	(4)	(since C++17)

Sorts the elements in the range [first, last) in non-descending order. 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 any of the following conditions is satisfied, the behavior is undefined:

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

first, last	-	the range of elements to sort
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
- `RandomIt` must meet the requirements of LegacyRandomAccessIterator.
- `Compare` must meet the requirements of Compare.

[edit] Complexity

Given $\scriptsize N$ $N$ as last - first:

1,2)

O(N\cdotlog(N))

comparisons using operator<(until C++20)std::less{}(since C++20).

3,4)

O(N\cdotlog(N))

applications of the comparator comp.

[edit] 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.

[edit] Possible implementation

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

[edit] Notes

Before LWG713, the complexity requirement allowed sort() to be implemented using only Quicksort, which may need $\scriptsize O(N^2)$ $O(N 2)$ comparisons in the worst case.

Introsort can handle all cases with $\scriptsize O(N \cdot \log(N))$ $O(N\cdotlog(N))$ comparisons (without incurring additional overhead in the average case), and thus is usually used for implementing sort().

libc++ has not implemented the corrected time complexity requirement until LLVM 14.

[edit] Example

Run this code

#include <algorithm>
#include <array>
#include <functional>
#include <iostream>
#include <string_view>
 
int main()
{
    std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
 
    auto print = [&s](std::string_view const rem)
    {
        for (auto a : s)
            std::cout << a << ' ';
        std::cout << ": " << rem << '\n';
    };
 
    std::sort(s.begin(), s.end());
    print("sorted with the default operator<");
 
    std::sort(s.begin(), s.end(), std::greater<int>());
    print("sorted with the standard library compare function object");
 
    struct
    {
        bool operator()(int a, int b) const { return a < b; }
    }
    customLess;
 
    std::sort(s.begin(), s.end(), customLess);
    print("sorted with a custom function object");
 
    std::sort(s.begin(), s.end(), [](int a, int b)
                                  {
                                      return a > b;
                                  });
    print("sorted with a lambda expression");
}

Output:

0 1 2 3 4 5 6 7 8 9 : sorted with the default operator<
9 8 7 6 5 4 3 2 1 0 : sorted with the standard library compare function object
0 1 2 3 4 5 6 7 8 9 : sorted with a custom function object
9 8 7 6 5 4 3 2 1 0 : sorted with a lambda expression

[edit] 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 713	C++98	the $\scriptsize O(N \cdot \log(N))$ $O(N\cdotlog(N))$ time complexity was only required on the average	it is required for the worst case

[edit] See also

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

@@ Line 3: / Line 3: @@
 {{dcl begin}}
 {{dcl header|algorithm}}
-{{dcl rev multi|num=1|until1=c++20|dcl1=
+{{dcl|num=1|notes={{mark constexpr since c++20}}|
 template< class RandomIt >
 void sort( RandomIt first, RandomIt last );
-|dcl2=
-template< class RandomIt >
-constexpr void sort( RandomIt first, RandomIt last );
 }}
-{{dcl|since=c++17|num=2|1=
+{{dcl|num=2|since=c++17|
 template< class ExecutionPolicy, class RandomIt >
 void sort( ExecutionPolicy&& policy,
             RandomIt first, RandomIt last );
 }}
-{{dcl rev multi|num=3|until1=c++20|dcl1=
+{{dcl|num=3|notes={{mark constexpr since c++20}}|
 template< class RandomIt, class Compare >
 void sort( RandomIt first, RandomIt last, Compare comp );
-|dcl2=
-template< class RandomIt, class Compare >
-constexpr void sort( RandomIt first, RandomIt last, Compare comp );
 }}
-{{dcl|since=c++17|num=4|1=
+{{dcl|num=4|since=c++17|
 template< class ExecutionPolicy, class RandomIt, class Compare >
 void sort( ExecutionPolicy&& policy,
-            RandomIt first, RandomIt last, Compare comp );}}
+            RandomIt first, RandomIt last, Compare comp );
+}}
 {{dcl end}}
-Sorts the elements in the range {{tt|[first, last)}} in non-descending order. The order of equal elements is not guaranteed to be preserved.
+Sorts the elements in the range {{range|first|last}} in non-descending order. The order of equal elements is not guaranteed to be preserved.
-A sequence is sorted with respect to a comparator {{tt|comp}} if for any iterator {{tt|it}} pointing to the sequence and any non-negative integer {{tt|n}} such that {{tt|it + n}} is a valid iterator pointing to an element of the sequence, {{c|comp(*(it + n), *it)}} (or {{c|*(it + n) < *it}}) evaluates to {{tt|false}}.
+@1@ Elements are {{rlp|/#Requirements|sorted}} with respect to {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}}.
-@1@ Elements are compared using {{tt|operator<}}.
+@3@ Elements are sorted with respect to {{c|comp}}.
-@3@ Elements are compared using the given binary comparison function {{tt|comp}}.
-@2,4@ Same as {{v|1,3}}, but executed according to {{tt|policy}}. {{cpp/algorithm/parallel overload precondition|plural=true}}
+@2,4@ Same as {{v|1,3}}, but executed according to {{c|policy}}.
+@@ {{cpp/algorithm/parallel overload precondition|plural=yes}}
+If any of the following conditions is satisfied, the behavior is undefined:
+{{rev begin}}
+{{rev|until=c++11|
+* The type of {{c|*first}} is not {{named req|Swappable}}.
+}}
+{{rev|since=c++11|
+* {{tt|RandomIt}} is not {{named req|ValueSwappable}}.
+* The type of {{c|*first}} is not {{named req|MoveConstructible}}.
+* The type of {{c|*first}} is not {{named req|MoveAssignable}}.
+}}
+{{rev end}}
 ===Parameters===
@@ Line 42: / Line 50: @@
 {{par cmp ord|comp|p1=RandomIt}}
 {{par hreq}}
-{{par req named|RandomIt|RandomAccessIterator|ValueSwappable}}
+{{par req named|RandomIt|RandomAccessIterator}}
-{{par req named deref|RandomIt|MoveConstructible|MoveAssignable}}
 {{par req named|Compare|Compare}}
 {{par end}}
-===Return value===
-(none)
 ===Complexity===
-{{rev begin}}
+Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|last - first}}:
-{{rev|until=c++11|
+@1,2@ {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N·log(N))}} comparisons using {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}}.
-{{math|O(N&middot;log(N))}}, where {{c|1=N = std::distance(first, last)}} comparisons on average.
+@3,4@ {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N·log(N))}} applications of the comparator {{c|comp}}.
-}}
-{{rev|since=c++11|
-{{math|O(N&middot;log(N))}}, where {{c|1=N = std::distance(first, last)}} comparisons.
-}}
-{{rev end}}
 ===Exceptions===
-{{cpp/algorithm/parallel_exceptions_reporting_behavior|singular=no}}
+{{cpp/algorithm/parallel exceptions reporting behavior|singular=no}}
 ===Possible implementation===
 See also the implementations in [https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L1950 libstdc++] and [https://github.com/llvm/llvm-project/blob/e7fc254875ca9e82b899d5354fae9b5b779ff485/libcxx/include/__algorithm/sort.h#L264 libc++].
+===Notes===
+Before {{wg21|LWG713}}, the complexity requirement allowed {{tt|sort()}} to be implemented using only {{enwiki|Quicksort}}, which may need {{mathjax-or|\(\scriptsize O(N^2)\)|O(N{{su|p=2}})}} comparisons in the worst case.
+{{enwiki|Introsort}} can handle all cases with {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N·log(N))}} comparisons (without incurring additional overhead in the average case), and thus is usually used for implementing {{tt|sort()}}.
+libc++ has not implemented the corrected time complexity requirement [https://reviews.llvm.org/D113413 until LLVM 14].
 ===Example===
@@ Line 71: / Line 77: @@
 |code=
 #include <algorithm>
-#include <functional>
 #include <array>
+#include <functional>
 #include <iostream>
 #include <string_view>
@@ Line 78: / Line 84: @@
 int main()
 {
-     std::array<int, 10> s = {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
+     std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
      auto print = [&s](std::string_view const rem)
      {
@@ Line 86: / Line 92: @@
          std::cout << ": " << rem << '\n';
      };
      std::sort(s.begin(), s.end());
      print("sorted with the default operator<");
      std::sort(s.begin(), s.end(), std::greater<int>());
      print("sorted with the standard library compare function object");
      struct
      {
@@ Line 101: / Line 107: @@
      std::sort(s.begin(), s.end(), customLess);
      print("sorted with a custom function object");
      std::sort(s.begin(), s.end(), [](int a, int b)
                                    {
@@ Line 114: / Line 120: @@
 8 7 6 5 4 3 2 1 0 : sorted with a lambda expression
 }}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=713|std=C++98|before=the {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N·log(N))}} time complexity was only required on the average|after=it is required for the worst case}}
+{{dr list end}}
 ===See also===

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)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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Difference between revisions of "cpp/algorithm/sort"

Latest revision as of 22:49, 1 April 2024

Contents

[edit] Parameters