Problem printing the output

Question

i'm writing a getClassMean() which is going to compute the Mean score of the whole class. in the following myLines is a ArrayList object that stores each String from my input file, which look something like:
Stevens, Frank, Z.; 87, 86, 92, 78, 86
Franklin, Susan, A.; 98, 88, 94, 98, 95
Bowers, Brett, B.; 85, 78, 83, 84, 89
This is the method in the Mean class:
protected ArrayList<Double> classGrades = new ArrayList<Double>();
public double getClassMean(){
            String gb;
            String[] array, grades;
            double mean;
            double score;
            for(String l : myLines) {
                array = l.split("; ");
                gb = array[1];    //gb --grade block
                grades = gb.split(", ");
                for(String scoreStr : grades ) {
                    score = Double.parseDouble(scoreStr);
                    classGrades.add(score);
            double sum = 0.0;
            for(double d : classGrades) {sum += d;}
            mean = sum / classGrades.size();
            return mean;
        }and the Driver program has this:
Mean m = new Mean();
double classMean = mean.getClassMean();
System.out.println(classMean);but the output print something i don't understand:
NaN
can someone tell me what's going on? thanks!

Answer 1

NaN is "Not a Number" it comes from doing operations with undefined results, like 0.0 / 0.0 .
Print out sum and classGrades.size() before doing that division, then trace backwards (add more print statements if you need to) to find your bug.