Algs4-2.2.23-1用实验评估正文中所提到的归并排序与三项改进的效果
2.2.23改进。用实验评估正文中所提到的归并排序的三项改进(请见练习2.2.11)的效果,并比较正文中实现的归并和练习2.2.10所实现的归并之间的性能。根据经验给出应该在何时为子数组切换到插入排序。
1)用实验评估正文中所提到的归并排序与三项改进(请见练习2.2.11)的效果。
通过以下实验得出,三项改进后的归并排序与正文中的归并排序要快,两者用时之比见下图。
import java.util.Arrays;
public class E2d2d23d1
{
public static double time (String alg,Double[] a)
{
Stopwatch timer =new Stopwatch();
if(alg.equals("Merge")) Merge.sort(a);
if(alg.equals("Merge2")) E2d2d11d1.sort(a);
return timer.elapsedTime();
}
public static double timeRandomInput(String alg,int N,int T)
{
double total =0.0;
Double[] a=new Double[N];
for (int t=0;t<T;t++)
{
for (int i=0;i<N;i++)
a[i]=StdRandom.uniform();
total+=time(alg,a);
}
return total/T;
}//end timeRandomInput
public static void main(String[] args)
{
int N=Integer.parseInt(args[0]);
int T=Integer.parseInt(args[1]);
StdOut.printf("For %d random Doubles %d times sort\n",N,T);
double t1=timeRandomInput("Merge",N,T);
double t2=timeRandomInput("Merge2",N,T);
StdOut.printf("Merge spend time=%.2f Merge2 spendtime=%.2f Merge2/Merge rate=%.2f\n",t1,t2,t2/t1);
}
}
public class E2d2d11d1
{
private static int subArrayLenTrunONInsertionSort=16;
public static void sort(Comparable[] a)
{
int len=a.length;
Comparable[] aux=new Comparable[len];
for(int i=0;i<len;i++)
aux[i]=a[i];
sort(a,aux,0,a.length-1);
}
private static void sort(Comparable[] a,Comparable[] aux,int lo,int hi)
{
if ((hi-lo+1)<=subArrayLenTrunONInsertionSort)
{
insertionSort(a,lo,hi);
}
else
{
int mid=lo+(hi-lo)/2;
sort(aux,a,lo,mid);
sort(aux,a,mid+1,hi);
if(!less(aux[mid+1],aux[mid]))
{
for(int i=lo;i<=hi;i++)
a[i]=aux[i];
return;
}
merge(aux,a,lo,mid,hi);
}
}
private static void merge(Comparable[] a,Comparable[] aux,int lo,int mid,int hi)
{
int i=lo;
int j=mid+1;
for(int k=lo;k<=hi;k++)
{
if (i>mid) aux[k]=a[j++];
else if (j>hi) aux[k]=a[i++];
else if (less(a[j],a[i])) aux[k]=a[j++];
else aux[k]=a[i++];
}
}
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 insertionSort(Comparable[] a,int lo,int hi)
{
for (int i=lo+1;i<=hi;i++)
for (int j=i;j>lo && less(a[j],a[j-1]);j--)
exch(a,j,j-1);
}
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;
}
}
import java.util.Arrays;
public class Merge
{
private static Comparable[] aux;
public static void sort(Comparable[] a)
{
aux=new Comparable[a.length];
sort(a,0,a.length-1);
}
public static void sort(Comparable[] a,int lo,int hi)
{
if (hi<=lo) return;
int mid=lo+(hi-lo)/2;
sort(a,lo,mid);
sort(a,mid+1,hi);
merge(a,lo,mid,hi);
}
public static void merge(Comparable[] a,int lo,int mid,int hi)
{
int i=lo,j=mid+1;
for (int k=lo;k<=hi;k++)
aux[k]=a[k];
for(int k=lo;k<=hi;k++)
if (i>mid) a[k]=aux[j++];
else if (j>hi) a[k]=aux[i++];
else if (less(aux[j],aux[i])) a[k]=aux[j++];
else a[k]=aux[i++];
}
private static boolean less(Comparable v,Comparable w)
{ return v.compareTo(w)<0;}
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;
}
}