Simple math to java practice

Question

The assignment is to:

Create the java statement that will produce f(x) = 2x3 - 8x2 + x + 16.
Write a Java program that will find the extrema (both high and low) for the function in part 1 across the closed interval (-1, 3).

I don't know what I am doing wrong here exactly but I am getting an infinite number of 9.09.09 and so on.

import java.util.*;
import java.math.*;

public class Extrema
{
   public static void main(String[] args)
   {
      double step = 0.00001;
      double x = -1.0;
      double xn = 3;
      double highest = calc(x);
      while (x <= xn)
      {
         double f = calc(x);
         if (f > highest)
         {
            highest = f;
         }
         step++;
         System.out.print(highest);
      }
   }
   public static double calc(double x)
   {
      double f = 2*Math.pow(x, 1/3) - 8*Math.pow(x, 2) + x + 16;
      return f;
   }
}

neither `x` nor `xn` change their values inside the loop, so it will run forever, and always calculate the same `x` — Numeron, Apr 08 '14 at 04:09
If you want x(cubed) why are you doing pow(x,1/3) .. that's cubed-root of x. you're supposed to increase x not step. The step is supposed to remain constant and the variable climbs up or down the step. — anu, Apr 08 '14 at 04:15
I strongly recommend you use a debugger to help you with problems like this. It would have shown you many of the elementary mistakes that you have made. — Dawood ibn Kareem, Apr 08 '14 at 04:17
@Sotirios It's possible that OP doesn't understand the significance of your comment. Go back and write it again! :-) — Dawood ibn Kareem, Apr 08 '14 at 04:18
[This `1/3` is integer division.](http://stackoverflow.com/questions/7220681/division-of-integers-in-java) — Sotirios Delimanolis, Apr 08 '14 at 04:21
Oh, wow, @Sotirios, I didn't expect you to take me seriously! — Dawood ibn Kareem, Apr 08 '14 at 06:05
The easiest way to evaluate this cubic function is `f(x)=((2*x- 8)*x+1)*x+16`, see Horner scheme. Thus completely avoiding any calls to logarithms and exponential functions. — Lutz Lehmann, Apr 08 '14 at 16:18

merlin2011 · Accepted Answer · 2014-04-08T04:17:25.903

0

Add x+=step at the end of (inside) the while loop.

import java.util.*;
import java.math.*;

public class Extrema
{
   public static void main(String[] args)
   {
      double step = 0.00001;
      double x = -1.0;
      double xn = 3;
      double highest = calc(x);
      while (x <= xn)
      {
         double f = calc(x);
         if (f > highest)
         {
            highest = f;
         }
         x += step;
      }
     System.out.println(highest);
   }
   public static double calc(double x)
   {
      double f = 2*Math.pow(x, 3) - 8*Math.pow(x, 2) + x + 16;
      return f;
   }
}

This outputs: 16.03175629885453 on my system.

edited Apr 08 '14 at 04:17

answered Apr 08 '14 at 04:11

merlin2011

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After the first round, that will skip some values of `x`, since `step` is also increasing. – Numeron Apr 08 '14 at 04:12
@Numeron, I am aware of that, but I was not sure what the intent was. – merlin2011 Apr 08 '14 at 04:13
@Numeron, I have edited it to match his top-level description of what he's trying to do. – merlin2011 Apr 08 '14 at 04:16
@merlin2011 Thank you so much! To do the lowest, I'd do it while x >= xn, right? – user3500131 Apr 08 '14 at 04:25
@user3500131, The loop only helps you move from x = -1 to x = 3. If you want to do lowest, you should think about how you are doing highest (in the if statement) and change that to get the smallest value instead of the largest. – merlin2011 Apr 08 '14 at 04:27

Simple math to java practice

1 Answers1