查找arrayList中位数的问题

Question

I keep getting The type of the expression must be an array type but it resolved to ArrayList<Double> The arrayList is pulling numbers from my Test class. I made the if to determine if the arrayList has an even amount of values in it or an odd, for I could use the two different ways to determine the median. But I'm not able to get the median formula to work.

public class Data {

private ArrayList<Double> sets;

public Data(double[] set) {
    this.sets = new ArrayList<Double>();
    for (double i : set) {
        this.sets.add(i);
    }
}

public double getMedian(){
    Collections.sort(sets);

    double middle = sets.size()/2;
        if (sets.size()%2 == 1) {
           middle = (sets[sets.size()/2] + sets[sets.size()/2 - 1])/2;
        } else {
            middle = sets[sets.size() / 2];
        }
      return middle;
}

Answer 1

The problem is on the line where you find the middle:

middle = (sets[sets.size()/2] + sets[sets.size()/2 - 1])/2;

You can use [index] notation only with arrays. You need to use getter/setter methods to access the elements of an ArrayList. This should work:

middle = (sets.get(sets.size()/2) + sets.get(sets.size()/2 - 1))/2;

Answer 2

You are using arrayList not an array, so ArrayList's get(int index) method must be used to access data. This is used like this :

middle = (sets.get(sets.size()/2) +sets.get((sets.size()/2)-1) )/2;

Answer 3

The problem is that, if you have an array with 4 elements [0..3], to find the median you just do 4/2 - 1 = 1 and index 1 is the median of array. Now if array is 5, you just do 5/2 = 2 and 2 is the median.

public double getMedian() {
    Collections.sort(sets);
    int middle = sets.size() / 2;
    middle = middle > 0 && middle % 2 == 0 ? middle - 1 : middle;
    return sets.get(middle);
}

Answer 4

Make sure in your if statement you check if it's equal to 0.

public double getMedian(){
    Collections.sort(sets);

    double middle = sets.size()/2;
        if (sets.size()%2 == 0) {
           middle = (sets.get(sets.size()/2) + sets.get(sets.size()/2 - 1))/2;
        } else {
            middle = sets[sets.size() / 2];
        }
      return middle;
}