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How to edit the array within a method?

So this method is called differentiate, and its purpose is to return a Poly object which consists of an array of doubles, this array should contain the coefficients of the differentiated polynomial for example if provided a poly object that contains an array with [2.0, 3.0, 2.0], the method will return [4, 3, 0] because 2x^2 + 3x^1 + 2.0‘s coefficients are those.

public static Poly polyObject;

public static String differentiate(Poly polyObject) {
    double[] array = polyObject.getDoubleArray();
    int counterVariable = array.length - 1;
    for (int i = 0; i < array.length; i++) {
        array[i] = array[i] * counterVariable;
        counterVariable--;
    }
}

Not sure what to do from here in order to change the array’s coefficients.

Answer

You can get the result by apply Horner’s method. This also shows the resulting coefficients.

  • original equation = y = 2x^3 + 6x^2 +4x + 3
  • after derivation = y' = 6x^2 + 12x + 4
  • given x = 3, the result is 54 + 36 + 4 = 94

To solve using Horner's Method let result = 0

  • result = result * x + 6 = 6 ( exp = 2)
  • result = result * x + 12 = 30 (exp = 1)
  • result = result * x + 4 = 94 (exp = 0) - done!
double[] coefs = { 2., 6., 4., 3 };
int exp = coefs.length-1;
double result = 0;
int i = 0;
int x = 3; // the value to be solve
while(i < exp) {
    coefs[i] *= (exp-i);
    result = result * x + coefs[i++];
}


// y = 2x^3 + 6x^2 +4x + 3
// After derivation. coefs = 6, 12, 4
// y' = 6x^2 + 12x + 4    =    54 + 36 + 4
coefs = Arrays.copyOf(coefs,coefs.length-1);
System.out.println(Arrays.toString(coefs));
System.out.println("result = " + result);

Prints

[6.0, 12.0, 4.0]
result = 94.0