如何对Java文本文件进行排序，以便获得中位数？

Question

I have the code below so far. I know the median is wrong because I've placed numbers in it and what I really want is for the program to extract these from the file on its own since the numbers may change. I am not sure how to have the program get the 2 numbers in order to retrieve and calculate the median. Please help. I am very new to this and it has taken me all day to get this far!

    package trials;

import java.util.Scanner;
import java.io.IOException;
import java.io.File;

public class trials2 {

public static void main(String[] args) throws IOException {
    // This is a Scanner object that reads from the keyboard
    Scanner in = new Scanner(System.in);

    // The following is set to find the file
    System.out.println("Please enter the name of your data file: ");
    String fileName = in.next();

    // The file is then to be scanned
    Scanner fileToRead = new Scanner(new File(fileName));

    // This loop finds the contents within the file
    double sum = 0;
    int numStudents = 0;
    double maxVal = 0, minVal = 0;
    boolean bFirstTime = true;
    double currVal;
    while (fileToRead.hasNext()) { 
        if (fileToRead.hasNextDouble()) {
            numStudents++;
            currVal = fileToRead.nextDouble();

            // The following will find the maximum and minimum values within the file
            if (bFirstTime) {
                maxVal = currVal;
                minVal = currVal;
                bFirstTime = false;
            } else {
                maxVal = Math.max(maxVal,currVal);
                minVal = Math.min(minVal, currVal);
            }

            sum += currVal;
        } else {
            fileToRead.next();
        }   
    }
   // Prints out comments and results
   System.out.println("***Welcome to the Exam Statistics Program!!***");
   System.out.println();
   System.out.println("Minimum = " + minVal);
   System.out.println("Maximum = " + maxVal);   
   System.out.println("Average score: " + sum/numStudents);
   System.out.println();
   System.out.println("Number of scores by letter grade: ");
   System.out.println();       
   System.out.println("There are " + numStudents + " scores");
}
}

Answer 1

There are a few steps to doing this.

1. At the beginning of your program, create an ArrayList<Double> for storing your values in.

2. Within your main loop, use the list's add method to add each value as you read it.

3. At the end of your loop, Use Collections.sort to sort your list.

4. Use the following logic to work out the median.

   • If the size of the list is zero, then there's no median.
   • If the size of the list is odd, then the median is the value at position size() / 2 of the list.
   • If the size of the list is even, then the median is the average of the value at position size() / 2 - 1 and size() / 2 of the list.

I deliberately haven't given you code, because I think you're enjoying learning how to do this for yourself. But feel free to post a comment if you need more detail on any particular step.