STLalgorithm算法partition_point,stable_partition(44)


STLalgorithm算法partition_point,stable_partition(44)

partition_point原型:

std::partition_point

template <class ForwardIterator, class UnaryPredicate>
  ForwardIterator partition_point (ForwardIterator first, ForwardIterator last,
                                   UnaryPredicate pred);

该函数返回范围内第一个不符合pred(即返回值为false)的元素的迭代器.

范围内的元素应该已经进行过划分,即使用partition进行划分。否则可能会出错!

该函数优化了查找的次数。

其行为类似如下(类似于二分查找):

template <class ForwardIterator, class UnaryPredicate>
  ForwardIterator partition_point (ForwardIterator first, ForwardIterator last,
                                   UnaryPredicate pred)
{
  auto n = distance(first,last);
  while (n>0)
  {
    ForwardIterator it = first;
    auto step = n/2;
    std::advance (it,step);
    if (pred(*it)) { first=++it; n-=step+1; }
    else n=step;
  }
  return first;
}

一个简单的例子:

#include <iostream>  
#include <vector>  
#include <array>  
#include <algorithm>  
using namespace std;  
int main(){  
    vector<int> vi{8,5,10,7,1,3,5,8,9,13};  
    cout<<"vi=";  
    for_each(vi.begin(),vi.end(),[](int i){cout<<i<<" ";});  
    cout<<endl;  
    if(is_partitioned(vi.begin(),vi.end(),[](int n){return n%2==0;}))  
        cout<<"v1 is a partitioned!"<<endl;  
    else  
        cout<<"v1 not a partitioned!"<<endl;  
	
	auto it=partition(vi.begin(),vi.end(),[](int n){return n%2==0;});  
	cout<<"after  auto it=partition(vi.begin(),vi.end(),[](int n){return n%2==0;}); "<<endl;
	cout<<"vi=";  
   for_each(vi.begin(),vi.end(),[](int i){cout<<i<<" ";});  
	cout<<endl;
	if(is_partitioned(vi.begin(),vi.end(),[](int n){return n%2==0;}))  
        cout<<"v1 is a partitioned!"<<endl;  
    else  
        cout<<"v1 not a partitioned!"<<endl;  
	cout<<"*it="<<*it<<endl;
	
	auto it2=partition_point(vi.begin(),vi.end(),[](int n){return n%2==0;});
	cout<<"auto it2=partition_point(vi.begin(),vi.end(),[](int n){return n%2==0;});"<<endl;
	cout<<"it2="<<*it2<<endl;
    
}  

运行结果:

返回值和partition是一样的。




stable_partition原型:

std::stable_partition

template <class BidirectionalIterator, class UnaryPredicate>
  BidirectionalIterator stable_partition (BidirectionalIterator first,
                                          BidirectionalIterator last,
                                          UnaryPredicate pred);

该函数对范围内的元素进行稳定划分。

返回值为第一个返回false的元素的迭代器,如果没有,则返回last.

和partition是有区别的,当然在数值上是看不出来的。区别为:

稳定和不稳定,这对数值没有什么用,但对于多属性的对象就有用了。
如以下组合序列,按括号外的数排序,括号内是另一个属性,
1(1), 3(2), 5(3), 5(4), 7(5), 9(6) , 7(7)
稳定排序后
1(1), 3(2), 5(3), 5(4), 7(5), 7(7) , 9(6)
两个5和两个7的相对位置不变,

不稳定排序后,可以是5(4), 5(3)或7(7), 7(5)

感谢:http://zhidao.baidu.com/link?url=wpPsrq1rg5IQioAZ_afz4Qd5drgWSUfiw-GU1GcK_ur3cwNlLCgzwVIR5q0sEdsmS_H0ajKWM97wVsJcgK6uUuH-cl9_ZZzE4wRdTMi1M7e


一个简单的例子:

#include <iostream>  
#include <vector>  
#include <array>  
#include <algorithm>  
using namespace std;  
int main(){  
    vector<int> vi{8,5,10,7,1,3,5,8,9,13};  
    cout<<"vi=";  
    for_each(vi.begin(),vi.end(),[](int i){cout<<i<<" ";});  
    cout<<endl;  
    if(is_partitioned(vi.begin(),vi.end(),[](int n){return n%2==0;}))  
        cout<<"v1 is a partitioned!"<<endl;  
    else  
        cout<<"v1 not a partitioned!"<<endl;  
	
	auto it=stable_partition(vi.begin(),vi.end(),[](int n){return n%2==0;});  
	cout<<"after  auto it=stable_partition(vi.begin(),vi.end(),[](int n){return n%2==0;}); "<<endl;
	cout<<"vi=";  
   for_each(vi.begin(),vi.end(),[](int i){cout<<i<<" ";});  
	cout<<endl;
	if(is_partitioned(vi.begin(),vi.end(),[](int n){return n%2==0;}))  
        cout<<"v1 is a partitioned!"<<endl;  
    else  
        cout<<"v1 not a partitioned!"<<endl;  
	cout<<"*it="<<*it<<endl;
    
}  

运行截图:

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//写的错误或者不好的地方请多多指导,可以在下面留言或者点击左上方邮件地址给我发邮件,指出我的错误以及不足,以便我修改,更好的分享给大家,谢谢。

转载请注明出处:http://blog.csdn.net/qq844352155

author:天下无双

Email:coderguang@gmail.com

2014-9-22

于GDUT

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