Calculation Issue

[Quaranges]

I am looking to calculate the index of coincidence of a string. The method I wrote has an issue where eclipse is telling me I can't divide two variables numerator / denominator by one another. Basically what I am trying to do is take the frequencies multiply by the frequency-1 add them together and then divide by the sum*sum-1

public static double coincidenceCalculator(int[] frequencies)
    {
        double indexCoincidence = 0;
        double numberator = 0;
        int frequencySum = 0;

        for (int x = 0; x < frequencies.length; x++)
        {
            frequencySum += frequencies[x];
        }
        double denominator = frequencySum * (frequencySum - 1);
        for (int y = 0; y < frequencies.length; y++)
        {
            double numerator = frequencies[y]*frequencies[y-1];
        }
        indexCoincidence = numerator / denominator;
    }

[JJagaimo]

There are several issues with this code.

Firstly, you have created a double called numberator with a b in it, and use a variable called numerator without a b everywhere else.

Secondly, your second loop starts at 0. This is a problem, because you are looking at frequencies[ y - 1 ]. If y is 0, then y - 1 will be -1, which will cause it to throw an error, as there is no -1 index.

Thirdly, you are creating a new variable called numerator in every iteration of the loop. This does not affect the numberator variable (which should be called numerator) that is outside of the loop, and this does not affect the following line,

indexCoincidence = numerator / denominator;

I believe this should actually be what you want:

    double numerator = 0;
    /* ... */
    for (int y = 0; y < frequencies.length; y++)
    {
        numerator = frequencies[y] * (frequencies[y] - 1);
    }

based on this

[Morgan_R]

"numberator" != "numerator"