C++ Algorithm partition()用法及代碼示例

C++ 算法 partition() 函數用於根據其參數中提到的給定謂詞(條件)對元素進行分區。如果容器已分區，則此函數返回 true，否則返回 false。

用法

template <class BidirectionalIterator, class UnaryPredicate>
BidirectionalIterator partition (BidirectionalIterator first,
BidirectionalIterator last, UnaryPredicate pred);	      //until C++ 11

template <class ForwardIterator, class UnaryPredicate>
ForwardIterator partition (ForwardIterator first,
ForwardIterator last, UnaryPredicate pred);	     //Since C++ 11

參數

first: 一個雙向迭代器，指向要分區的範圍內的第一個元素。

last:一個雙向迭代器，指向要分區的範圍中過去的最後一個元素。

pred: 用戶定義的一元謂詞函數，它定義要對元素進行分類時要滿足的條件。

返回值

此函數返回一個迭代器，指向不滿足謂詞條件的範圍的第一個元素。

複雜度

複雜度在 [first, last) 範圍內是線性的：將 pred 應用於每個元素，並可能交換其中的一些元素。

數據競爭

範圍 [first, last) 中的對象被修改。

異常

如果元素交換或迭代器上的操作引發異常，則此函數將引發異常。

請注意，無效參數會導致未定義的行為。

例子1

讓我們看一個簡單的例子來演示 partition() 的使用：

#include <iostream>     // std::cout
#include <algorithm>    // std::partition
#include <vector>       // std::vector

using namespace std;

bool IsOdd (int i) { return (i%2)==1; }

int main () {
  vector<int> myvector;

  // set some values:
  for (int i=1; i<10; ++i) myvector.push_back(i); // 1 2 3 4 5 6 7 8 9

  vector<int>::iterator bound;
  bound = partition (myvector.begin(), myvector.end(), IsOdd);

  // print out content:
  cout << "odd elements:";
  for (vector<int>::iterator it=myvector.begin(); it!=bound; ++it)
    cout << ' ' << *it;
  cout << '\n';

  cout << "even elements:";
  for (vector<int>::iterator it=bound; it!=myvector.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';

  return 0;
}

輸出：

odd elements:1 9 3 7 5
even elements:6 4 8 2

在上麵的例子中，從 1 到 9 的元素被劃分為偶數和奇數元素。

例子2

讓我們看另一個簡單的例子：

#include <vector>  
#include <algorithm>  
#include <iostream>  
  
bool greater5 ( int value ) {  
   return value >5;  
}  
  
int main( ) {  
   using namespace std;  
   vector <int> v1, v2;  
   vector <int>::iterator Iter1, Iter2;  
  
   int i;  
   for ( i = 0 ; i <= 10 ; i++ )  
   {  
      v1.push_back( i );  
   }  
   random_shuffle( v1.begin( ), v1.end( ) );  
  
   cout << "Vector v1 is ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")." << endl;  
  
   // Partition the range with predicate greater10  
   partition ( v1.begin( ), v1.end( ), greater5 );  
   cout << "The partitioned set of elements in v1 is:( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")." << endl;  
   
   return 0;
}

輸出：

Vector v1 is ( 4 10 7 8 0 5 2 1 6 9 3 ).
The partitioned set of elements in v1 is:( 9 10 7 8 6 5 2 1 0 4 3 ).

例子3

讓我們看另一個使用 partition() 函數快速排序向量元素的簡單示例：

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <forward_list>
 
template <class ForwardIt>
 void quicksort(ForwardIt first, ForwardIt last)
 {
    if(first == last) return;
    auto pivot = *std::next(first, std::distance(first,last)/2);
    ForwardIt middle1 = std::partition(first, last, 
                         [pivot](const auto& em){ return em < pivot; });
    ForwardIt 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:\n    ";
    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:\n    ";
    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:\n    ";
    for(int n:fl) std::cout << n << ' ';
    std::cout << '\n';  
 
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "Sorted using quicksort:\n    ";
    for(int fi:fl) std::cout << fi << ' ';
    std::cout << '\n';
    
    return 0;
}

輸出：

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

示例 4

讓我們看另一個簡單的例子：

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
  vector<int> v = {1, 2, 3, 4, 5};
  
  cout<<"Before Partition:";
    for_each(v.begin(), v.end(), [](int v) {
   cout << v << " ";
  });


  auto pred = [](int x) { return x % 2 == 0; };

  // Divide it into an even group and an odd group 
  partition(v.begin(), v.end(), pred);
  cout<<"\nAfter partition:";
  for_each(v.begin(), v.end(), [](int x) {
   cout << x << " ";
  });

  cout<<"\n\nIs it partitioned?"<<endl;
  // Is it divided into an even group and an odd group?  
  if (is_partitioned(v.begin(), v.end(), pred)) {
    cout << "Yes, it is partitioned" << endl;
  }
  else {
    cout << "No, it is not partitioned" << endl;
  }
  
  return 0;
}

輸出：

Before Partition:1 2 3 4 5 
After partition:4 2 3 1 5 

Is it partitioned?
Yes, it is partitioned

相關用法

注：本文由純淨天空篩選整理自 C++ Algorithm partition()。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。