CODEWITHSUNDEEP
javaarraysloopsinitializationdouble
h4nkt4nk
h4nkt4nk

Reputation: 15

Initializing an array of doubles java

I cannot for the life of me understand why this code isn't working. I am creating a function to evaluate a polynomial at all of the points over a given x interval, at a frequency specified by the user. My underlying Polynomial code is irrelevent to the problem. My problem is filling my array with the values I am obtaining from Horner's method.

public double[] evalAt(double s, double f, int n) {
    double[] resultArray = new double[n];
    double h =((f - s)/(n));
    if(s==f) {
        double tempResult = 0;
        for (int i = this.degree; i >= 0; i--) {
            tempResult = tempResult * s + this.terms[i].getCoefficient();
        }
        resultArray[0] = tempResult;
    } else {
        int counter = 0;
        for(double i = s; i <= f; i=i+h) {
            double tempResult = 0;
            for (int j = this.degree; j >= 0; j--) {
                tempResult = tempResult * i + this.terms[j].getCoefficient();
            }
            System.out.println("Counter: " + counter + " Result @ Counter: " + tempResult);
            resultArray[counter++] = tempResult;
        }
        System.out.println(resultArray[0]);
        System.out.println(resultArray[1]);
        System.out.println(resultArray[2]);
        System.out.println(resultArray[3]);
    }
    return resultArray;
}

I have a print statement in there to show me what the values of the counter and the temporary result are. then I use the resultArray['index'] = 'value'; syntax. When I run this, I see that the counter and the value are exactly as I want them, however when I check the array with the four print statements at the bottom, nothing happens. My inputs are 0,10,10:

Output:

Counter: 0 Result @ Counter: 0.0
Counter: 1 Result @ Counter: 1.0
Counter: 2 Result @ Counter: 4.0
Counter: 3 Result @ Counter: 9.0
Counter: 4 Result @ Counter: 16.0
Counter: 5 Result @ Counter: 25.0
Counter: 6 Result @ Counter: 36.0
Counter: 7 Result @ Counter: 49.0
Counter: 8 Result @ Counter: 64.0
Counter: 9 Result @ Counter: 81.0
Counter: 10 Result @ Counter: 100.0

Upvotes: 1

Views: 21133

Answers (2)

Cameron Skinner
Cameron Skinner

Reputation: 54356

I strongly suspect it is throwing an ArrayIndexOutOfBoundsException which is getting swallowed outside this method.

You are creating an array of size n, in this case 10, but your counter goes from zero to f (also 10) which is a total of 11 elements. So there's an off-by-one in at least one of those.

Look through your code and find the bit that says

catch (Exception e) {
}

and change it to

catch (Exception e) {
    e.printStackTrace();
}

to help catch this kind of error more easily.

Upvotes: 3

AntonyM
AntonyM

Reputation: 1604

Unfortunately I can't post comments yet as this isn't a concrete answer but I have checked and run your code and it runs fine for me. Substituting in a fixed coefficient and degree show that the bracketing is all fine and that you aren't (as I suspected) reinitialising the resultArray variable in an inner scope.

Is this a direct copy/paste of the problematic code that you have?

Typically when I have an issue like this it is nothing to do with the assignment but instead it is because 'resultsArray' is actually 2 different variables that I have erroneously allocated but which I am receiving no errors for because of polymorphism.

Upvotes: 1

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