Groovy example source code file (spectralnorm.java)

This example Groovy source code file (spectralnorm.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Groovy tags/keywords

a, a, approximate, atav, atv, atv, av, av, decimalformat, multiplyatav, multiplyatav, multiplyatv, multiplyav, numberformat, text

The Groovy spectralnorm.java source code

/* 
 The Great Computer Language Shootout
 http://shootout.alioth.debian.org/
 
 contributed by Java novice Jarkko Miettinen
 modified ~3 lines of the original C#-version 
 by Isaac Gouy
 */
 
import java.text.DecimalFormat;
import java.text.NumberFormat; 

public class spectralnorm
{
	
	private static final NumberFormat formatter = new DecimalFormat("#.000000000");
	
	public static void main(String[] args) {
		int n = 100;
		if (args.length > 0) n = Integer.parseInt(args[0]);
		
		System.out.println(formatter.format(new spectralnorm().Approximate(n)));
	}
	
	private final double Approximate(int n) {
		// create unit vector
		double[] u = new double[n];
		for (int i=0; i<n; i++) u[i] =  1;
		
		// 20 steps of the power method
		double[] v = new double[n];
		for (int i=0; i<n; i++) v[i] = 0;
		
		for (int i=0; i<10; i++) {
			MultiplyAtAv(n,u,v);
			MultiplyAtAv(n,v,u);
		}
		
		// B=AtA         A multiplied by A transposed
		// v.Bv /(v.v)   eigenvalue of v
		double vBv = 0, vv = 0;
		for (int i=0; i<n; i++) {
			vBv += u[i]*v[i];
			vv  += v[i]*v[i];
		}
		
		return Math.sqrt(vBv/vv);
	}
	
	
	/* return element i,j of infinite matrix A */
	private final double A(int i, int j){
		return 1.0/((i+j)*(i+j+1)/2 +i+1);
	}
	
	/* multiply vector v by matrix A */
	private final void MultiplyAv(int n, double[] v, double[] Av){
		for (int i=0; i<n; i++){
			Av[i] = 0;
			for (int j=0; j<n; j++) Av[i] += A(i,j)*v[j];
		}
	}
	
	/* multiply vector v by matrix A transposed */
	private final void MultiplyAtv(int n, double[] v, double[] Atv){
		for (int i=0;i<n;i++){
			Atv[i] = 0;
			for (int j=0; j<n; j++) Atv[i] += A(j,i)*v[j];
		}
	}
	
	/* multiply vector v by matrix A and then by matrix A transposed */
	private final void MultiplyAtAv(int n, double[] v, double[] AtAv){
		double[] u = new double[n];
		MultiplyAv(n,v,u);
		MultiplyAtv(n,u,AtAv);
	}
}