算法导论第九章

最新推荐文章于 2022-08-11 13:09:10 发布

wang19890326

最新推荐文章于 2022-08-11 13:09:10 发布

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CC 4.0 BY-SA版权

本文链接：https://blog.youkuaiyun.com/wang19890326/article/details/11224455

本文介绍了一个基于模板的随机数生成器类Urand，并实现了一个选择算法SELECT，该算法用于寻找未排序数组中的第i小元素。通过划分和递归调用，实现了高效的查找过程。

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#include<iostream>         //算法导论第九章
#include<bitset>
#include<ctime>
using namespace std;

template <size_t UpperBound> class Urand{      //生成随机数  
	bitset<UpperBound> used;  
public:  
    Urand(){ srand(time(0)); }  
    double operator() ();  
};  

template<size_t UpperBound>  
inline double Urand<UpperBound>::operator()()  
{  
    if(used.count() == UpperBound)  
        used.reset();  
    size_t newval;  
    while(used[newval = rand() % UpperBound])  
       ;  
    used[newval] = true;  
    return newval*0.1;  
}  


int partition( double darr[], int p, int q)   //划分  
{  
   
    int groupnum = (q-p+1)/5;
	int lastgroupnum = (q-p+1)-groupnum*5;
    
	int num = (lastgroupnum==0) ? groupnum : groupnum+1;
    double *Mid = new double[num];   //中位数
	int *Marr = new int[num];        //中位数标志位
    int i = 0;   //循环标志位
	int j = 0;
	int h = 0;
	int k = 0;
	int w = 0;
	int y = 0;
	int z = 0;
	
    for( i=0; i<groupnum; i++)
	{
		for( j=i*5+1+p; j<i*5+5+p; j++)
		{   
			double indexOne = darr[j];
			if( darr[j-1]> indexOne )
			{
				for( h = j-1; darr[h]>indexOne&&h>=i*5; h--)
				{
					darr[h+1]=darr[h];
				}
				darr[h+1]=indexOne;
			}

		
		}
		Mid[i] = darr[(i*5+p+i*5+4+p)/2];
		Marr[i] = (i*5+p+i*5+4+p)/2;   //记住标志
	}

    if( lastgroupnum != 0)
	{
	   for( k=groupnum*5+1+p; k<groupnum*5+lastgroupnum+p; k++)
	   {   
		double indexTwo = darr[k];
		if( darr[k-1]>indexTwo )
		{
			for( w=k-1; darr[w]>indexTwo&&w>=groupnum*5; w--)
			{
                  darr[w+1]=darr[w];
			}
				darr[w+1]=indexTwo;
		}
	
	   }
	   Mid[groupnum] = darr[(groupnum*5+p+groupnum*5+lastgroupnum+p)/2];
       Marr[groupnum] = (groupnum*5+p+groupnum*5+lastgroupnum+p)/2;   //记住标志位
    }
	
	groupnum = ( lastgroupnum==0 ) ? groupnum : groupnum+1;
	for( y=1; y<groupnum; y++)    //中位数排序
	{   
		double indexThree = Mid[y];
		int temp = Marr[y];
		if( Mid[y-1]>indexThree )
		{
		  for( z=y-1; Mid[z]>indexThree&&z>=0; z-- )
		  {
                 Mid[z+1]=Mid[z];
				 Marr[z+1]=Marr[z];
		  }
		      Mid[z+1]=indexThree;
			  Marr[z+1]=temp;
			 
		}
	}
    double dtemp = darr[Marr[groupnum/2]];
	darr[Marr[groupnum/2]] = darr[p];
	darr[p] = dtemp;

	double x = darr[p];  
    int start = p;
    for( int r=p+1; r<=q; r++)  
	{  
       if( darr[r]<=x )  
      {  
           start++;  
           swap(darr[start], darr[r]);  
       }  
	}  
    swap(darr[start], darr[p]);  
    return start;
}  

double SELECT( double dArr[], int p, int q, int i)   //select函数
{  
    if( p==q )  
    {  
       return dArr[p];  
    }  
    int r = partition( dArr, p, q);  
    int k = r-p+1;  
    if( i==k )  
    {  
        return dArr[r];  
    }  
    else if( i<k )  
    {  
        return SELECT( dArr, p, r-1, i);  
    }  
    else  
    {  
       return SELECT( dArr, r+1, q, i-k);  
	}  
}  


int main()
{   
	Urand<23> u;
	double darr[23] = { 0.0 };
	for( int i=0; i<23; i++)
	{
		darr[i]=u();
	}
    cout<<SELECT(darr, 0, 22, 11)<<endl;
      
	
	return 0;
}