Bisection Method: evaluate a polynomial function

Question

I am implementing the bisection method, and as a first step I need to evaluate the polynomial function, but I do not get the correct result.

The polynomial used is 3(x^2)+7(x)+1
with x=2, the result should be 27

public static double evaluaFx(int []coef, int grade, int x) { 
   //int x viene siendo los x0,xf.xmenter code here
   double Fx=0;
   //System.out.println("grade"+grade+"x"+x);
   //for (int i = grade; i >=0; i--) {
   //   System.out.println(coef[i]);
   //}
   for (int i = grade; i>=0; i--) {
      Fx= Math.pow((coef[i]*x), grade);
      // System.out.println(Fx+"mas"+"("+coef[i]+x+") a la grado"+grade);
   }
   return Fx;
}

What's wrong with above algorithm ?

Could you add the arguments and expected output? – Darshan Mehta Feb 26 '17 at 00:23 — Darshan Mehta, Feb 26 '17 at 00:23
I used the next polynomial f(x)=3x^2+7x+1 with f(x)=27 – Frank R Feb 26 '17 at 00:27 — Frank R, Feb 26 '17 at 00:27

score 1 · Answer 1 · answered Mar 01 '17 at 11:28

Several things are wrong in your algorithm:

Formula for one grade (step i) should be:

 // for instance, if i=2, you want 3 * x^2 which translate to
 Fx = coef[i]*Math.pow(x, i);

Then in the for loop you don't add up successive grade computations, loosing at each step the previous result. You should do something like (note the += instead of =)

 Fx += coef[i]*Math.pow(x, i);

Bisection Method: evaluate a polynomial function

1 Answers1