Difference between revisions of "cpp/algorithm/inplace merge"

Latest revision as of 04:12, 10 July 2024

Defined in header `<algorithm>`
template< class BidirIt > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last );	(1)	(constexpr since C++26)
template< class ExecutionPolicy, class BidirIt > void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last );	(2)	(since C++17)
template< class BidirIt, class Compare > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last, Compare comp );	(3)	(constexpr since C++26)
template< class ExecutionPolicy, class BidirIt, class Compare > void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last, Compare comp );	(4)	(since C++17)

Merges two consecutive sorted ranges [first, middle) and [middle, last) into one sorted range [first, last).

1) If [first, middle) or [middle, last) is not sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.

3) If [first, middle) or [middle, last) is not sorted with respect to comp, the behavior is undefined.

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)

This merge function is stable, which means that for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

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

[first, middle) or [middle, last) is not a valid range.
The output range overlaps with [first, middle) or [middle, last).

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

first	-	the beginning of the first sorted range
middle	-	the end of the first sorted range and the beginning of the second
last	-	the end of the second sorted 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 BidirIt can be dereferenced and then implicitly converted to both of them.
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.
- `Compare` must meet the requirements of Compare.

[edit] Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1) Exactly

N-1

comparisons using operator<(until C++20)std::less{}(since C++20) if enough additional memory is available,

O(N\cdotlog(N))

comparisons otherwise.

2)

O(N\cdotlog(N))

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

3) Exactly

N-1

applications of the comparison function comp if enough additional memory is available,

O(N\cdotlog(N))

applications otherwise.

4)

O(N\cdotlog(N))

applications of the comparison function 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 the implementations in libstdc++ and libc++.

[edit] Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

Feature-test macro	Value	Std	Feature
`__cpp_lib_constexpr_algorithms`	202306L	(C++26)	constexpr inplace merging (1), (3)

[edit] Example

The following code is an implementation of merge sort.

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
template<class Iter>
void merge_sort(Iter first, Iter last)
{
    if (last - first > 1)
    {
        Iter middle = first + (last - first) / 2;
        merge_sort(first, middle);
        merge_sort(middle, last);
        std::inplace_merge(first, middle, last);
    }
}
 
int main()
{
    std::vector<int> v{8, 2, -2, 0, 11, 11, 1, 7, 3};
    merge_sort(v.begin(), v.end());
    for (const auto& n : v)
        std::cout << n << ' ';
    std::cout << '\n';
}

Output:

-2 0 1 2 3 7 8 11 11

[edit] See also

merge	merges two sorted ranges (function template) [edit]
sort	sorts a range into ascending order (function template) [edit]
stable_sort	sorts a range of elements while preserving order between equal elements (function template) [edit]
ranges::inplace_merge (C++20)	merges two ordered ranges in-place (niebloid)[edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|inplace_merge}}
-{{cpp/algorithm/sidebar}}
+{{cpp/algorithm/navbar}}
-{{ddcl list begin}}
+{{dcl begin}}
-{{ddcl list header | algorithm}}
+{{dcl header|algorithm}}
-{{ddcl list item | num=1 |
+{{dcla|num=1|constexpr=c++26|
-template< class BidirectionalIterator >
+template< class BidirIt >
-void inplace_merge( BidirectionalIterator first,
+void inplace_merge( BidirIt first, BidirIt middle, BidirIt last );
-                    BidirectionalIterator middle,
-                    BidirectionalIterator last );
 }}
-{{ddcl list item | num=2 |
+{{dcl|num=2|since=c++17|
-template< class BidirectionalIterator, class Compare>
+template< class ExecutionPolicy, class BidirIt >
-void inplace_merge( BidirectionalIterator first,
+void inplace_merge( ExecutionPolicy&& policy,
-                     BidirectionalIterator middle,
+                    BidirIt first, BidirIt middle, BidirIt last );
-                     BidirectionalIterator last,
+}}
+{{dcla|num=3|constexpr=c++26|
+template< class BidirIt, class Compare >
+void inplace_merge( BidirIt first, BidirIt middle, BidirIt last,
+                     Compare comp );
+}}
+{{dcl|num=4|since=c++17|
+template< class ExecutionPolicy, class BidirIt, class Compare >
+void inplace_merge( ExecutionPolicy&& policy,
+                     BidirIt first, BidirIt middle, BidirIt last,
                      Compare comp );
 }}
-{{ddcl list end}}
+{{dcl end}}
-Merges two consecutive sorted ranges {{tt|[first, middle)}} and {{tt|[middle, last)}} into one sorted range {{tt|[first, last)}}. The order of equal elements is guaranteed to be preserved. The first version uses {{cpp|operator<}} to compare the elements, the second version uses the given comparison function {{tt|comp}}.
+Merges two consecutive sorted ranges {{range|first|middle}} and {{range|middle|last}} into one sorted range {{range|first|last}}.
-===Parameters===
+@1@ If {{range|first|middle}} or {{range|middle|last}} is not {{rlp|/#Requirements|sorted}} with respect to {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}}, the behavior is undefined.
-{{param list begin}}
-{{param list item | first | the beginning of the first sorted range }}
-{{param list item | middle | the end of the first sorted range and the beginning of the second }}
-{{param list item | last | the end of the second sorted range }}
-{{param list cmp | comp | p1=BidirectionalIterator}}
-{{param list end}}
-===Return value===
+@3@ If {{range|first|middle}} or {{range|middle|last}} is not sorted with respect to {{c|comp}}, the behavior is undefined.
-{{return none}}
+@2,4@ Same as {{v|1,3}}, but executed according to {{c|policy}}.
+@@ {{cpp/algorithm/parallel overload precondition|plural=yes}}
+This merge function is stable, which means that for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).
+If any of the following conditions is satisfied, the behavior is undefined:
+* {{range|first|middle}} or {{range|middle|last}} is not a [[cpp/iterator#Ranges|valid range]].
+* The output range overlaps with {{range|first|middle}} or {{range|middle|last}}.
+{{rev begin}}
+{{rev|until=c++11|
+* The type of {{c|*first}} is not {{named req|Swappable}}.
+}}
+{{rev|since=c++11|
+* {{tt|BiditIt}} 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===
+{{par begin}}
+{{par|first|the beginning of the first sorted range}}
+{{par|middle|the end of the first sorted range and the beginning of the second}}
+{{par|last|the end of the second sorted range}}
+{{par exec pol}}
+{{par cmp ord|comp|p1=BidirIt}}
+{{par hreq}}
+{{par req named|BidirIt|BidirectionalIterator}}
+{{par req named|Compare|Compare}}
+{{par end}}
 ===Complexity===
+Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}}:
-{{tt|(last - first) - 1}} comparisons if enough additional memory available or {{tt|N log(N)}} where {{tt|1=N = last - first}} if no additional memory is available.
+@1@ Exactly {{mathjax-or|\(\scriptsize N-1\)|N-1}} comparisons using {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}} if enough additional memory is available, {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N⋅log(N))}} comparisons otherwise.
+@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{}<!---->}}}}.
+@3@ Exactly {{mathjax-or|\(\scriptsize N-1\)|N-1}} applications of the comparison function {{c|comp}} if enough additional memory is available, {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N⋅log(N))}} applications otherwise.
+@4@ {{mathjax-or|\(\scriptsize O(N \cdot \log(N))\)|O(N⋅log(N))}} applications of the comparison function {{c|comp}}.
+===Exceptions===
+{{cpp/algorithm/parallel exceptions reporting behavior|singular=no}}
+===Possible implementation===
+See the implementations in [https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L2508 libstdc++] and [https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L4452 libc++].
+===Notes===
+This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.
+{{feature test macro|__cpp_lib_constexpr_algorithms|{{c/core|constexpr}} inplace merging {{vl|1}}, {{vl|3}}|value=202306L|std=C++26}}
 ===Example===
-{{example cpp
+{{example
- | std::inplace_merge allows for a simple recursive implementation of merge sort
+|The following code is an implementation of merge sort.
- | code=
+|code=
-#include <vector>
-#include <iostream>
 #include <algorithm>
+#include <iostream>
+#include <vector>
-template<typename Iter>
+template<class Iter>
 void merge_sort(Iter first, Iter last)
 {
-  if (last - first > 1)
+    if (last - first > 1)
-  {
+    {
-    Iter middle = first + (last - first) / 2;
+        Iter middle = first + (last - first) / 2;
-    merge_sort(first, middle);
+        merge_sort(first, middle);
-    merge_sort(middle, last);
+        merge_sort(middle, last);
-    std::inplace_merge(first, middle, last);
+        std::inplace_merge(first, middle, last);
-  }
+    }
 }
@@ Line 59: / Line 108: @@
      std::vector<int> v{8, 2, -2, 0, 11, 11, 1, 7, 3};
      merge_sort(v.begin(), v.end());
-     for(auto n : v) {
+     for (const auto& n : v)
          std::cout << n << ' ';
-    }
      std::cout << '\n';
 }
- | output=
+|output=
 -2 0 1 2 3 7 8 11 11
 }}
 ===See also===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/algorithm/dcl list partial_sort}}
+{{dsc inc|cpp/algorithm/dsc merge}}
-{{dcl list template | cpp/algorithm/dcl list sort}}
+{{dsc inc|cpp/algorithm/dsc sort}}
-{{dcl list template | cpp/algorithm/dcl list stable_sort}}
+{{dsc inc|cpp/algorithm/dsc stable_sort}}
-{{dcl list end}}
+{{dsc inc|cpp/algorithm/ranges/dsc inplace_merge}}
+{{dsc end}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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Difference between revisions of "cpp/algorithm/inplace merge"

Latest revision as of 04:12, 10 July 2024

Contents

[edit] Parameters