从文本文件读取问题

Question

I am trying to create a program in Java that calculates the average cost of winning the lottery and saves that average to reference when it runs again (my goal is to be able to create a more accurate outcome every time i run it). The average successfully saves to my txt file, but when I run the program it uses 0 as the previous average every time. Any help is greatly appreciated! Thanks! (Number of times it runs can be changed by changing the 'runs' variable)

public class WinningTheLottery
{
public static final int SIZE = 5;

public static void main(String[] args)
{
    int[] winningNums = new int[SIZE];
    int[] guessNums = new int[SIZE];
    int spent, runs = 1;
    double oldAvg = 0;
    double newAvg;
    int totalSpent = 0;
    NumberFormat currency = NumberFormat.getCurrencyInstance();
    try
    {
        Scanner fileScanner = new Scanner(new File("average.txt"));
        PrintWriter fileWriter = new PrintWriter(new File("average.txt"));
        while (fileScanner.hasNextDouble())
        {
            oldAvg = fileScanner.nextDouble();
        }
        for (int i = 0; i < runs; i++)
        {
            spent = 0;
            randomlyAssignedNumbers(winningNums);
            // Arrays.toString(nameOfArray) => built in method to print an array
            System.out.println("\n[" + (i+1) + "] Todays winning numbers:\n" + Arrays.toString(winningNums).replace("[", "").replace("]", ""));
            do
            {
                randomlyAssignedNumbers(guessNums);
                spent++;
            } while (howManyCorrect(winningNums, guessNums) < 5);
            System.out.println("After spending " + currency.format(spent) + ", you won the Fantasy 5 lottery $75,000 prize!");
            totalSpent += spent;
        }
        newAvg = ((totalSpent/runs) +oldAvg)/2;
        fileWriter.println(newAvg);
        System.out.println("\nAverage Cost to win the Lottery: " + currency.format(newAvg)
                        + "\n(Previous Average: " + currency.format(oldAvg) + ")");
        fileScanner.close();
        fileWriter.close();
    }
    catch(IOException e)
    {
        System.out.println(e.getMessage());
    }

}

public static void randomlyAssignedNumbers(int[] anyArray)
{
    Random rng = new Random();
    for (int i = 0; i < anyArray.length; i++)
    {
        anyArray[i] = rng.nextInt(36) + 1;
    }
}

public static int howManyCorrect(int[] a1, int[] a2)
{
    if (a1.length != a2.length)
        return -1;
    int count = 0;
    for (int i = 0; i < a1.length; i++)
    {
        if (a1[i] == a2[i])
            count++;
    }
    return count;
}
}

Answer 1

Try not constructing the PrintWriter until after you read the previous value. As noted in the Javadoc the file will be truncated by creating the PrintWriter.

Answer 2

Your code is opening the file in overwrite mode, default of many programming languages, before scanner can read the content, thus deleting the content before fileScanner.hasNextDouble() read. Move your PrintWriter instantiation after Scanner read, and it will work.

Note: To open the file in append mode, which I don't think you need here. But in Java you use new FileWriter("average.txt", true) and then wrap a PrintWriter around it.