Java で平方根 (sqrt) を求めるコードのデモ

/* 
 * Algorithms.java 
 * 
 * Created on: 2013.12.03 
 *   Author: Wendy 
 */ 

/*eclipse std kepler, jdk 1.7*/ 
 
public class Algorithms  
{ 
  public static double sqrt(double c) 
  { 
    if(c<0) return Double.NaN; //NaN: not a number 
    double err = 1e-15; //极小值 
    double t = c; 
    while (Math.abs(t-c/t) > err*t) //t^2接近c, 防止小数 
      t = (c/t + t)/2.0; 
    return t; 
  } 
   
  public static double cbrt(double c) 
  { 
    boolean b = (c>0) ? true : false; //保存c的符号 
    c = (c>0) ? c : -c; 
    double err = 1e-15; 
    double t = c; 
    while(Math.abs(t*t-c/t) > err*t) 
      t = (c/(t*t)+t)/2.0; 
    t = (b) ? t : -t; 
    return t; 
  } 
   
  public static void main(String[] args)  
  { 
    double r = sqrt(4.0); 
    System.out.println("sqrt(4.0) = " + r); 
    double rc = cbrt(-27.0); 
    System.out.println("cbrt(9.0) = " + rc); 
  } 
}

sqrt(4.0) = 2.0 
cbrt(9.0) = -3.0