Round a double to 3 significant figures

Question

Does anybody know how I can round a double value to 3 significant figures like the examples on this website

http://www.purplemath.com/modules/rounding2.htm

Answer 1

double d = ...; BigDecimal bd = new BigDecimal(d); bd = bd.round(new MathContext(3)); double rounded = bd.doubleValue(); 

Answer 2

If you want to do it by hand:

import java.lang.Math;  public class SigDig {    public static void main(String[] args) {     System.out.println("   -123.456   rounded up   to 2 sig figures is " + sigDigRounder(-123.456, 2,  1));     System.out.println("     -0.03394 rounded down to 3 sig figures is " + sigDigRounder(-0.03394, 3, -1));     System.out.println("    474       rounded up   to 2 sig figures is " + sigDigRounder(474, 2,  1));     System.out.println("3004001       rounded down to 4 sig figures is " + sigDigRounder(3004001, 4, -1));   }    public static double sigDigRounder(double value, int nSigDig, int dir) {      double intermediate = value/Math.pow(10,Math.floor(Math.log10(Math.abs(value)))-(nSigDig-1));      if(dir > 0)      intermediate = Math.ceil(intermediate);     else if (dir< 0) intermediate = Math.floor(intermediate);     else             intermediate = Math.round(intermediate);      double result = intermediate * Math.pow(10,Math.floor(Math.log10(Math.abs(value)))-(nSigDig-1));      return(result);    } } 

The above method rounds a double to a desired number of significant figures, handles negative numbers, and can be explicitly told to round up or down