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Difference between revisions of "cpp/algorithm/partition"

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< cpp‎ | algorithm
m
(Added LWG issue #2150 DR (part 6/12), and uses {{mark constexpr since c++20}}.)
 
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{{cpp/algorithm/navbar}}
 
{{cpp/algorithm/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | algorithm}}
+
{{dcl header|algorithm}}
−
{{dcl rev begin}}
+
{{dcl|num=1|notes={{mark constexpr since c++20}}|
−
{{dcl | until=c++11 |
+
template< class ForwardIt, class UnaryPred >
−
template< class BidirIt, class UnaryPredicate >
+
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );
−
BidirectionalIterator partition( BidirIt first, BidirIt last,
+
−
                                UnaryPredicate p );
+
 
}}
 
}}
−
{{dcl | since=c++11 |
+
{{dcl|num=2|since=c++17|
−
template< class ForwardIt, class UnaryPredicate >
+
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
−
ForwardIt partition( ForwardIt first, ForwardIt last,
+
ForwardIt partition( ExecutionPolicy&& policy,
−
                    UnaryPredicate p );
+
                    ForwardIt first, ForwardIt last, UnaryPred p );
 
}}
 
}}
−
{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
−
Reorders the elements in the range {{tt|[first, last)}} in such a way that all elements for which the predicate {{tt|p}} returns {{c|true}} precede the elements for which predicate {{tt|p}} returns {{c|false}}. Relative order of the elements is not preserved.  
+
@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}}
 +
 
 +
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===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | first, last | the range of elements to reorder}}
+
{{par|first, last|the range of elements to reorder}}
−
{{par pred1 | p | if the element should be ordered before other elements | p1=ForwardIt}}
+
{{par exec pol}}
 +
{{par pred1|p|if the element should be ordered before other elements|p1=ForwardIt}}
 
{{par hreq}}
 
{{par hreq}}
−
{{par req concept | BidirIt | BidirectionalIterator}}
+
{{par req named|ForwardIt|ForwardIterator}}
−
{{par req concept | ForwardIt | ForwardIterator | ValueSwappable | notes=However, the operation is more efficient if {{tt|ForwardIt}} also satisfies the requirements of {{concept|BidirectionalIterator}}}}
+
{{par req named|UnaryPred|Predicate}}
−
{{par req concept | UnaryPredicate | Predicate}}
+
 
{{par end}}
 
{{par end}}
  
Line 33: Line 35:
  
 
===Complexity===
 
===Complexity===
−
Exactly {{c|last-first}} applications of the predicate and at most {{c|last-first}} swaps. If {{tt|ForwardIt}} meets the requirements of {{concept|BidirectionalIterator}} at most {{c|(last-first)/2}} swaps are done.
+
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.
 +
 
 +
===Exceptions===
 +
{{cpp/algorithm/parallel exceptions reporting behavior|singular=yes}}
  
 
===Possible implementation===
 
===Possible implementation===
−
{{eq fun | 1=
+
Implements overload {{v|1}} preserving C++11 compatibility.
−
template<class BidirIt, class UnaryPredicate>
+
{{eq fun|1=
−
BidirIt partition(BidirIt first, BidirIt last, UnaryPredicate p)
+
template<class ForwardIt, class UnaryPred>
 +
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred 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;
 
             ++first;
 
         }
 
         }
−
        if (first == last--) break;
+
      
−
        while ((first != last) && !p(*last)) {
+
−
            --last;
+
−
        }
+
−
        if (first == last) break;
+
−
        std::iter_swap(first++, last);
+
−
     }
+
 
     return first;
 
     return first;
 
}
 
}
−
 
 
}}
 
}}
  
−
===Example 1===
+
===Example===
 
{{example
 
{{example
−
|
+
|
−
| code=
+
|code=
 
#include <algorithm>
 
#include <algorithm>
−
#include <functional>
+
#include <forward_list>
 
#include <iostream>
 
#include <iostream>
 
#include <iterator>
 
#include <iterator>
 
#include <vector>
 
#include <vector>
  
−
bool is_even(int i) { return i % 2 == 0; }
+
template<class ForwardIt>
−
 
+
void quicksort(ForwardIt first, ForwardIt last)
−
int main()
+
 
{
 
{
−
     std::vector<int> v;
+
     if (first == last)
−
    for (int i = 0; i < 10; ++i) v.push_back(i);
+
        return;
 
      
 
      
−
     std::cout << "Original vector:\n    ";
+
     auto pivot = *std::next(first, std::distance(first, last) / 2);
−
     std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
+
     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);
 +
    });
 
      
 
      
−
     // Partition the vector
+
     quicksort(first, middle1);
−
    std::vector<int>::iterator p =
+
     quicksort(middle2, last);
−
        std::partition(v.begin(), v.end(), std::ptr_fun(is_even));
+
−
      
+
−
    std::cout << "\nPartitioned vector:\n    ";
+
−
    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
+
−
    std::cout << "\nBefore partition:\n    ";
+
−
    std::copy(v.begin(), p,      std::ostream_iterator<int>(std::cout, " "));
+
−
    std::cout << "\nAfter partition:\n    ";
+
−
    std::copy(p,        v.end(), std::ostream_iterator<int>(std::cout, " "));
+
 
}
 
}
−
| 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
 
−
Before partition:
 
−
    0 8 2 6 4
 
−
After partition:
 
−
    5 3 7 1 9
 
−
}}
 
  
−
===Example 2===
+
int main()
−
{{example
+
{
−
|
+
    std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
−
| code=
+
    std::cout << "Original vector: ";
−
#include <iterator>
+
    for (int elem : v)
−
#include <algorithm>
+
        std::cout << elem << ' ';
−
#include <forward_list>
+
−
#include <iostream>
+
−
 
+
−
template <class ForwardIt>
+
−
ForwardIt choose_pivot(ForwardIt first, ForwardIt last)
+
−
{
+
−
  return std::next(first, std::distance(first,last)/2);
+
−
}
+
−
 
+
−
template <class ForwardIt>
+
−
void quicksort(ForwardIt first, ForwardIt last)
+
−
{
+
−
    using T = typename std::iterator_traits<ForwardIt>::value_type;
+
 
      
 
      
−
     if(first == last) return;
+
     auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
 
      
 
      
−
     T pivot = *choose_pivot(first,last);
+
     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, " "));
 
      
 
      
−
     ForwardIt middle1 = std::partition(first, last,  
+
     std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
−
                        [pivot](const T& em){ return em < pivot; });
+
     std::cout << "\nUnsorted list: ";
−
     ForwardIt middle2 = std::partition(middle1, last,
+
    for (int n : fl)
−
                        [pivot](const T& em){ return !(pivot < em); });
+
        std::cout << n << ' ';
 
      
 
      
−
     quicksort(first, middle1);
+
     quicksort(std::begin(fl), std::end(fl));
−
    quicksort(middle2, last);
+
    std::cout << "\nSorted using quicksort: ";
−
}
+
    for (int fi : fl)
−
+
        std::cout << fi << ' ';
−
+
    std::cout << '\n';
−
int main()
+
−
{
+
−
  std::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
+
−
 
+
−
  quicksort(std::begin(fl), std::end(fl));
+
−
 
+
−
  for(int fi : fl) std::cout << fi << ' ';
+
−
  std::cout << '\n';
+
 
}
 
}
−
| output =
+
|p=true
−
-8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92  
+
|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  
 
}}
 
}}
 +
 +
===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}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/algorithm/dsc is_partitioned}}
+
{{dsc inc|cpp/algorithm/dsc is_partitioned}}
−
{{dsc inc | cpp/algorithm/dsc stable_partition}}
+
{{dsc inc|cpp/algorithm/dsc stable_partition}}
 +
{{dsc inc|cpp/algorithm/ranges/dsc partition}}
 
{{dsc end}}
 
{{dsc end}}
  
−
[[de:cpp/algorithm/partition]]
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
−
[[es:cpp/algorithm/partition]]
+
−
[[fr:cpp/algorithm/partition]]
+
−
[[it:cpp/algorithm/partition]]
+
−
[[ja:cpp/algorithm/partition]]
+
−
[[pt:cpp/algorithm/partition]]
+
−
[[ru:cpp/algorithm/partition]]
+
−
[[zh:cpp/algorithm/partition]]
+

Latest revision as of 00:08, 29 March 2024

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
partition
(C++11)  
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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.

Contents

[edit] Parameters

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 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·log(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

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