算法第四版2.1Elementary Sorts：习题2.1.27

import edu.princeton.cs.algs4.StdOut;

public class E2_1_27 {
    public static void main(String[]args){
        String alg1= "Shell";
        String alg2= "Selection";
        int N=128;
        int T=100;

        double prev1=SortCompare.timeRandomInput(alg1,N,T);//total for alg1
        double prev2=SortCompare.timeRandomInput(alg2,N,T);//total for alg2
        for (N=128*2;true;N+=N){
            double now1=SortCompare.timeRandomInput(alg1,N,T);//total for alg1
            double now2=SortCompare.timeRandomInput(alg2,N,T);//total for alg2
            StdOut.printf("N=%8d  shell's time=%7.4f  ratio=%4.1f" +
                          "  selection's time=%7.4f  ratio=%4.1f\n",N,now1/T,now1/prev1,now2/T,now2/prev2 );
            prev1=now1;
            prev2=now2;
        }
    }
}

SortCompare的实现：

import edu.princeton.cs.algs4.*;

public class SortCompare {
    public static void main(String[]args){
        StdOut.print("Please input algorithm 1 name: ");
        String alg1= StdIn.readString();
        StdOut.print("\nPlease input algorithm 2 name: ");
        String alg2=StdIn.readString();
        StdOut.print("\nPlease input N: ");
        int N=StdIn.readInt();
        StdOut.print("\nPlease input times T: ");
        int T=StdIn.readInt();

        double t1=timeRandomInput(alg1,N,T);//total for alg1
        double t2=timeRandomInput(alg2,N,T);//total for alg2
        //StdOut.printf("For %d random Double\n\t %s is %.1f times faster than %s\n",N,alg1,t2/t1,alg2);
        StdOut.printf("N=%d  T=%d\n\talg1=%s time=%.1f\n\talg2=%s time=%.1f  t1/t2=%.2f",N,T,alg1,t1,alg2,t2,t1/t2);
    }

    public static double time(String alg,Comparable[]a){
        Stopwatch timer=new Stopwatch();
        if (alg.toLowerCase().equals("Insertion".toLowerCase()))    Insertion.sort(a);
        if (alg.toLowerCase().equals("Selection".toLowerCase()))    Selection.sort(a);
        if (alg.toLowerCase().equals("Shell".toLowerCase()))        Shell.sort(a);


        return timer.elapsedTime();
    }
    public static double timeRandomInput(String alg,int N,int T){
        //Use alg to sort T random arrays of length N.
        double total=0.0;
        Double[]a=new Double[N];
        for (int t=0;t<T;t++){
            //Perform one experiment(generate and sort an array).
            for (int i=0;i<N;i++)
                a[i]= StdRandom.uniform();
            total+=time(alg,a);
        }
        return total;
    }
}

结果：