Difference between revisions of "cpp/algorithm/partition"

Latest revision as of 00:08, 29 March 2024

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

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.

first, last	-	the range of elements to reorder
policy	-	the execution policy to use. See execution policy for details.
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.

[edit] Return value

Iterator to the first element of the second group.

[edit] Complexity

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

1) Exactly

N

applications of p.

At most

N/2

swaps if ForwardIt meets the requirements of LegacyBidirectionalIterator, and at most

N

swaps otherwise.

2)

O(N)

applications of p.

O(N\cdotlog(N))

swaps.

[edit] 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.

[edit] 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;
}

[edit] 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](const auto& em)
    {
        return em < pivot;
    });
    auto middle2 = std::partition(middle1, last, [pivot](const auto& 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

[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 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

[edit] See also

is_partitioned (C++11)	determines if the range is partitioned by the given predicate (function template) [edit]
stable_partition	divides elements into two groups while preserving their relative order (function template) [edit]
ranges::partition (C++20)	divides a range of elements into two groups (niebloid)[edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|partition}}
-{{cpp/algorithm/sidebar}}
+{{cpp/algorithm/navbar}}
-{{ddcl list begin}}
+{{dcl begin}}
-{{ddcl list header | algorithm}}
+{{dcl header|algorithm}}
-{{ddcl list item | notes={{mark until c++11}}<br><br>{{mark since c++11}} |
+{{dcl|num=1|notes={{mark constexpr since c++20}}|
-template< class BidirectionalIterator, class UnaryPredicate >
+template< class ForwardIt, class UnaryPred >
-BidirectionalIterator partition( BidirectionalIterator first, BidirectionalIterator last,
+ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );
-                                 UnaryPredicate p );
-template< class ForwardIterator, class UnaryPredicate >
-ForwardIterator partition( ForwardIterator first, ForwardIterator last,
-                                 UnaryPredicate p );
 }}
-{{ddcl list end}}
+{{dcl|num=2|since=c++17|
+template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
+ForwardIt partition( ExecutionPolicy&& policy,
+                     ForwardIt first, ForwardIt last, UnaryPred p );
+}}
+{{dcl end}}
+@1@ Reorders the elements in the range {{range|first|last}} in such a way that all elements for which the predicate {{c|p}} returns {{c|true}} precede all elements for which predicate {{c|p}} returns {{c|false}}. Relative order of the elements is not preserved.
+@2@ Same as {{v|1}}, but executed according to {{c|policy}}.
+@@ {{cpp/algorithm/parallel overload precondition}}
-Reorders the elements in the range {{tt|[first, last)}} in such a way that all elements for which the predicate {{tt|p}} returns {{cpp|true}} precede the elements for which predicate {{tt|p}} returns {{cpp|false}}. Relative order of the elements is not preserved.
+If {{rev inl|until=c++11|the type of {{c|*first}} is not {{named req|Swappable}}}}{{rev inl|since=c++11|{{tt|ForwardIt}} is not {{named req|ValueSwappable}}}}, the behavior is undefined.
 ===Parameters===
-{{param list begin}}
+{{par begin}}
-{{param list item | first, last | the range of elements to reorder}}
+{{par|first, last|the range of elements to reorder}}
-{{param list pred1 | p | if the element should be ordered before other elements | p1=ForwardIterator}}
+{{par exec pol}}
-{{param list end}}
+{{par pred1|p|if the element should be ordered before other elements|p1=ForwardIt}}
+{{par hreq}}
+{{par req named|ForwardIt|ForwardIterator}}
+{{par req named|UnaryPred|Predicate}}
+{{par end}}
 ===Return value===
+Iterator to the first element of the second group.
-Iterator to the first element of the second group
 ===Complexity===
+Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}}:
+@1@ Exactly {{mathjax-or|\(\scriptsize N\)|N}} applications of {{c|p}}.
+@@ At most {{mathjax-or|\(\scriptsize N/2\)|N/2}} swaps if {{tt|ForwardIt}} meets the requirements of {{named req|BidirectionalIterator}}, and at most {{mathjax-or|\(\scriptsize N\)|N}} swaps otherwise.
+@2@ {{mathjax-or|\(\scriptsize O(N)\)|O(N)}} applications of {{c|p}}.
+@@ {{mathjax-or|\(\scriptsize O(N \cdot log(N))\)|O(N·log(N))}} swaps.
-Exactly {{cpp|last-first}} applications of the predicate and at most {{cpp|(last-first)/2}} swaps if {{tt|ForwardIterator}} meets the requirements of {{concept|BidirectionalIterator}}, otherwise at most {{cpp|last-first}} swaps.
+===Exceptions===
+{{cpp/algorithm/parallel exceptions reporting behavior|singular=yes}}
 ===Possible implementation===
-{{eq fun cpp | 1=
+Implements overload {{v|1}} preserving C++11 compatibility.
-template<class BidirectionalIterator, class UnaryPredicate>
+{{eq fun|1=
-BidirectionalIterator partition(BidirectionalIterator first,
+template<class ForwardIt, class UnaryPred>
-                                BidirectionalIterator last,
+ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred p)
-                                UnaryPredicate p)
 {
-     while (1) {
+     first = std::find_if_not(first, last, p);
-         while ((first != last) && p(*first)) {
+    if (first == last)
+         return first;
+    for (auto i = std::next(first); i != last; ++i)
+        if (p(*i))
+        {
+            std::iter_swap(i, first);
              ++first;
          }
-        if (first == last--) break;
-        while ((first != last) && !p(*last)) {
-            --last;
-        }
-        if (first == last) break;
-        std::swap(*first++, *last);
-     }
      return first;
 }
 }}
 ===Example===
 {{example
- |
+|
- | code=
+|code=
 #include <algorithm>
-#include <functional>
+#include <forward_list>
 #include <iostream>
 #include <iterator>
 #include <vector>
-bool is_even(int i) { return i % 2 == 0; }
+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](const auto& em)
+    {
+        return em < pivot;
+    });
+    auto middle2 = std::partition(middle1, last, [pivot](const auto& em)
+    {
+        return !(pivot < em);
+    });
+    quicksort(first, middle1);
+    quicksort(middle2, last);
+}
 int main()
 {
-     std::vector<int> v;
+     std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
-     for (int i = 0; i < 10; ++i) v.push_back(i);
+    std::cout << "Original vector: ";
+     for (int elem : v)
+        std::cout << elem << ' ';
-     std::cout << "Original vector:\n    ";
+     auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
-    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
-     // Partition the vector
+     std::cout << "\nPartitioned vector: ";
-     std::vector<int>::iterator p =
+     std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
-        std::partition(v.begin(), v.end(), std::ptr_fun(is_even));
+    std::cout << "* ";
+    std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
-     std::cout << "\nPartitioned vector:\n    ";
+    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
-     std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
+     std::cout << "\nUnsorted list: ";
-    std::cout << "\nBefore partition:\n    ";
+     for (int n : fl)
-     std::copy(v.begin(), p,       std::ostream_iterator<int>(std::cout, " "));
+        std::cout << n << ' ';
-     std::cout << "\nAfter partition:\n    ";
-     std::copy(p,         v.end(), std::ostream_iterator<int>(std::cout, " "));
+    quicksort(std::begin(fl), std::end(fl));
+     std::cout << "\nSorted using quicksort: ";
+     for (int fi : fl)
+         std::cout << fi << ' ';
+    std::cout << '\n';
 }
+|p=true
+|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
 }}
-Possible output:
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=498|std=C++98|before={{tt|std::partition}} required {{c|first}} and<br>{{c|last}} to be {{named req|BidirectionalIterator}}|after=only required to be<br>{{named req|ForwardIterator}}}}
+{{dr list item|wg=lwg|dr=2150|std=C++98|before={{tt|std::partition}} was only required to place one element<br>satisfying {{c|p}} before one element not satisfying {{c|p}}|after=corrected the<br>requirement}}
+{{dr list end}}
-{{source
+===See also===
-| 1 =
+{{dsc begin}}
-Original vector:
+{{dsc inc|cpp/algorithm/dsc is_partitioned}}
-1 2 3 4 5 6 7 8 9
+{{dsc inc|cpp/algorithm/dsc stable_partition}}
-Partitioned vector:
+{{dsc inc|cpp/algorithm/ranges/dsc partition}}
-8 2 6 4 5 3 7 1 9
+{{dsc end}}
-Before partition:
-8 2 6 4
-After partition:
-3 7 1 9
-| lang=text}}
-{{todo | use possible_output parameter of the example template when implemented}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-===See also===
-{{dcl list begin}}
-{{dcl list template | cpp/algorithm/dcl list is_partitioned}}
-{{dcl list template | cpp/algorithm/dcl list stable_partition}}
-{{dcl list end}}

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Difference between revisions of "cpp/algorithm/partition"

Latest revision as of 00:08, 29 March 2024

Contents

[edit] Parameters