Algs4-2.3.29隨機化
2.3.29隨機化。用經驗性的研究對比隨機選擇切分元素和正文所述的一開始就將數組隨機化這兩處策略的效果。在子數組大小為M時進行切換，將大小為N的不重復數組排序，其中M=10、20和50,N=10^3、10^4、10^5和10^6。
答：由於生成的待排序數組本身已是隨機化，方式1在排序前打亂一次數組需要花費時間，所以方式1比方式2使用的排序時間要長一些,所以待排序數組本身已是隨機化時兩種方法比較性不是特別強。
public class E2d3d29
{
public static void main(String[] args)
{
int Nlen[]={1000,10000,100000,1000000};
int Mlen[]={10,20,50};
for(int Ni=0;Ni<Nlen.length;Ni++)
{
int N=Nlen[Ni];
for (int Mi=0;Mi<Mlen.length;Mi++)
{
int M=Mlen[Mi];
double s1Times=0.0;
double s2Times=0.0;
for (int time=0;time<10;time++)
{
Double[] a1=new Double[N];
Double[] a2=new Double[N];
for(int i=0;i<N;i++)
{
a1[i]=StdRandom.random();
a2[i]=a1[i];
}
Stopwatch timer1=new Stopwatch();
E2d3d29s1.sort(a1,M);
s1Times=s1Times+timer1.elapsedTime();
//
Stopwatch timer2=new Stopwatch();
E2d3d29s2.sort(a2,M);
s2Times=s2Times+timer2.elapsedTime();
}
StdOut.printf("N=%7d, M=%2d, shuffleElapsedTime=%.3f,
randomElapsedTime=%.3f,
shuffle-random=%.3f\n",N,M,s1Times/10,s2Times/10,(s1Times+s2Times)/10);
}
}//end for Ni
}//end main
}//end class
public class E2d3d29s1
{
public static void sort(Comparable[] a,int M)
{
StdRandom.shuffle(a);
sort(a,0,a.length-1,M);
}
private static void sort(Comparable[] a,int lo,int hi,int M)
{
if (hi-lo+1<M)
{
InsertSort(a,lo,hi);
return;
}
int j=partition(a,lo,hi);
sort(a,lo,j-1,M);
sort(a,j+1,hi,M);
}
private static int partition(Comparable[] a,int lo,int hi)
{
int i=lo,j=hi+1;
Comparable v=a[lo];
while(true)
{
while(less(a[++i],v)) if(i==hi) break;
while(less(v,a[--j])) if(j==lo) break;
if(i>=j) break;
exch(a,i,j);
}
exch(a,lo,j);
return j;
}
private static void InsertSort(Comparable[] a,int lo,int hi)
{
for (int i=lo+1;i<hi+1;i++)
{
for (int j=i;j>0 && less(a[j],a[j-1]);j--)
exch(a,j,j-1);
}
}
private static boolean less(Comparable v,Comparable w)
{ return v.compareTo(w)<0;}
private static void exch(Comparable[] a,int i,int j)
{
Comparable t=a[i];
a[i]=a[j];
a[j]=t;
}
private static void show(Comparable[] a)
{
for (int i=0;i<a.length;i++)
StdOut.print(a[i]+" ");
StdOut.println();
}
public static boolean isSorted(Comparable[] a)
{
for (int i=1;i<a.length;i++)
if(less(a[i],a[i-1])) return false;
return true;
}
public static void main(String[] args)
{
int N=Integer.parseInt(args[0]);
Double[] a=new Double[N];
for(int k=0;k<N;k++)
a[k]=StdRandom.random();
sort(a,10);
StdOut.println("isSorted="+isSorted(a));
}
}
public class E2d3d29s2
{
public static void sort(Comparable[] a,int M)
{
sort(a,0,a.length-1,M);
}
private static void sort(Comparable[] a,int lo,int hi,int M)
{
if (hi-lo+1<M)
{
InsertSort(a,lo,hi);
return;
}
int j=partition(a,lo,hi);
sort(a,lo,j-1,M);
sort(a,j+1,hi,M);
}
private static int partition(Comparable[] a,int lo,int hi)
{
int i=lo,j=hi+1;
exch(a,lo,StdRandom.uniform(lo,hi+1));
Comparable v=a[lo];
while(true)
{
while(less(a[++i],v)) if(i==hi) break;
while(less(v,a[--j])) if(j==lo) break;
if(i>=j) break;
exch(a,i,j);
}
exch(a,lo,j);
return j;
}
private static void InsertSort(Comparable[] a,int lo,int hi)
{
for (int i=lo+1;i<hi+1;i++)
{
for (int j=i;j>0 && less(a[j],a[j-1]);j--)
exch(a,j,j-1);
}
}
private static boolean less(Comparable v,Comparable w)
{ return v.compareTo(w)<0;}
private static void exch(Comparable[] a,int i,int j)
{
Comparable t=a[i];
a[i]=a[j];
a[j]=t;
}
private static void show(Comparable[] a)
{
for (int i=0;i<a.length;i++)
StdOut.print(a[i]+" ");
StdOut.println();
}
public static boolean isSorted(Comparable[] a)
{
for (int i=1;i<a.length;i++)
if(less(a[i],a[i-1])) return false;
return true;
}
public static void main(String[] args)
{
int N=Integer.parseInt(args[0]);
Double[] a=new Double[N];
for(int k=0;k<N;k++)
a[k]=StdRandom.random();
sort(a,10);
StdOut.println("isSorted="+isSorted(a));
}
}
Algs4-2.3.29隨機化