为什么我的变量没有在循环中更改？

Question

I've got a method that is meant to work out the group of elements of a 3D array with the highest combined value. I've got 3 nested loops that I'm using to go through my array, and as certain conditions are met I want to alter the variables. However none of the variables are being used. I ant to alter int y and int m to whatever iteration of the for loop it's on if sum exceeds total .

Thanks. Here is my code:

    public void wettestMonth(){
        double sum = 0;
        double total = 0;
        int y = 0;
        int m = 0;

        //cycle through each year and month
        for(int i = 0; i < 34; i++){
            for(int j = 0; j < 12; j++){
                //reset the current month to 0 after each month has been cycled through
                sum = 0;
                for(int k = 0; k < 31; k++){
                    //only add the record if the current entry is not null (-99.99)
                    if(sortedData[i][j][k] != -99.99){
                        sum += sortedData[i][j][k];
                    }
                    //if the current month is wetter than the wettest one, make the current month the new wettest one
                    if(sum > total){
                        total = sum;
                        y = i;
                        m = j;
                    }
                }
            }
        }

        JOptionPane.showMessageDialog(null, "The wettest month on record was " +m +y, "Wettest Month.", JOptionPane.PLAIN_MESSAGE);

    }

Edit, I just reconstructed it with while loops instead and I'm getting an out-of-bounds error on what appears to be the problem line, if(sortedData[i][j][k] != -99.99)

Edit 2, here is where I declare and initialise sortedData[][][]

public class GetData {

//initialises an array that holds 34 years, each with 12 months, each of which has 31 entries for reach day
public double[][][] sortedData = new double[34][12][31];

//initialises a new scanner named rainFile
private Scanner rainFile;

//method for opening the file
public void openFile() {

    try{
        //as the input for the scanner we use the rainfall file
        rainFile = new Scanner(new File("C:\\\\Users\\\\admin\\\\Documents\\\\NetBeansProjects\\\\110_term3\\\\WeatherDataFiles\\\\rainfall.txt"));
    }
    catch(Exception e){
        //if no file has been found a JOptionPane will display an error message telling the user to double-check the file path
        JOptionPane.showMessageDialog(null, "Check the file path is correct.", "No file found!", JOptionPane.ERROR_MESSAGE);
    }
}

//method for reading the file
public void readFile(){

    //ignore the first 3 lines in the data file
    String dump1 = rainFile.nextLine();
    String dump2 = rainFile.nextLine();
    String dump3 = rainFile.nextLine();

        //these nested for loops will dictate the current index of sortedData
        for(int i = 0; i < 34; i++){
            for(int j = 0; j < 12; j++){

                //ignores the year and month at the start of each line
                String dump4 = rainFile.next();
                String dump5 = rainFile.next();

                //this final nested for loop dictates the final index of sortedData
                for(int k = 0; k < 31; k++){

                    //asigns the current value of scanner rainFile to String a
                    String a = rainFile.next();

                    //converts the String a to a double type and then assigns it to the current index of sortedData
                    double dbl = Double.parseDouble(a);
                    sortedData[i][j][k] = dbl;
                }

            }
        }

    }

Answer 1

Have you tried printing out the sum for each month?

The most obvious possibility is that your sums are always less than 0 because of a buggy equality check.

For this line,

sortedData[i][j][k] != -99.99

It will be true unless the value is exactly what -99.99 rounds to. This may be unintended. For example, if you are constructing the value through floating point math somehow, you most likely won't get exactly the same value due to rounding errors. Furthermore, use of a weird sentinel value like this is error prone and less readable. You're better off using an obvious sentinel value like NaN if you can.

To see the problem, consider what would happen if your values are slightly different. Say, -99.99000001. Then after the first day, you already have a negative value. After a month, the sum will be approximately -3099.69000031, much less than 0. Since the sum is always negative, it is never better than the original total 0, so the best never gets updated.

You probably also want to move the update check outside of the day loop. This part looks like it's supposed to use the sum for the whole month, but you are running it with the partial sum of every day of the month. It won't actually cause incorrect results as long as the values being added are nonnegative (but they probably aren't due to the aforementioned bug), but you should still fix it.

                if(sum > total){
                    total = sum;
                    y = i;
                    m = j;
                }

Answer 2

I don't see anything wrong on your code.

Maybe the following condition never evals to true?

if(sortedData[i][j][k] != -99.99)

Answer 3

Carefully, reviewed the first code snippet that has been provided, it appears to me that you have been doing if check for the size inside the for loop, moved the for loop vars to the finals, added some further comments.

You could try and run your swing app through the Eclipse debug and see what results you get at each line in your application ?

Would expect it to work given correct input 3D array))

/**
* This method calculates the wettest month during the certain period of time.
*/
public void wettestMonth(){
    double sum = 0;
    double total = 0;
    int y = 0;
    int m = 0;
    final int numberOfYearsToCycleThrough = 34;
    final int numberOfMonthsToCycleThrough = 12;
    //cycle through each year and month
    for (int i = 0; i < numberOfYearsToCycleThrough; i++) {
        for (int j = 0; j < numberOfMonthsToCycleThrough; j++) {
            sum = 0;
            for (int k = 0; k < 31; k++){
                //only add the record if the current entry is not null (-99.99)
                if (sortedData[i][j][k] != null && sortedData[i][j][k] != -99.99) {
                   sum += sortedData[i][j][k];
                }
            }
        //if the current month is wetter than the wettest one, make the current month the new wettest one
             if (sum > total) {
                total = sum;
                y = i;
                m = j;
             }
         }
     }
     JOptionPane.showMessageDialog(null, "The wettest month on record was " +m +y, "Wettest Month.", JOptionPane.PLAIN_MESSAGE);
}