How to detect outliers in an ArrayList

Question

I\'m trying to think of some code that will allow me to search through my ArrayList and detect any values outside the common range of \"good values.\"

Example: 100 1

Answer 1

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List data = new ArrayList();
        data.add((double) 20);
        data.add((double) 65);
        data.add((double) 72);
        data.add((double) 75);
        data.add((double) 77);
        data.add((double) 78);
        data.add((double) 80);
        data.add((double) 81);
        data.add((double) 82);
        data.add((double) 83);
        Collections.sort(data);
        System.out.println(getOutliers(data));
    }

    public static List getOutliers(List input) {
        List output = new ArrayList();
        List data1 = new ArrayList();
        List data2 = new ArrayList();
        if (input.size() % 2 == 0) {
            data1 = input.subList(0, input.size() / 2);
            data2 = input.subList(input.size() / 2, input.size());
        } else {
            data1 = input.subList(0, input.size() / 2);
            data2 = input.subList(input.size() / 2 + 1, input.size());
        }
        double q1 = getMedian(data1);
        double q3 = getMedian(data2);
        double iqr = q3 - q1;
        double lowerFence = q1 - 1.5 * iqr;
        double upperFence = q3 + 1.5 * iqr;
        for (int i = 0; i < input.size(); i++) {
            if (input.get(i) < lowerFence || input.get(i) > upperFence)
                output.add(input.get(i));
        }
        return output;
    }

    private static double getMedian(List data) {
        if (data.size() % 2 == 0)
            return (data.get(data.size() / 2) + data.get(data.size() / 2 - 1)) / 2;
        else
            return data.get(data.size() / 2);
    }
}

Output: [20.0]

Explanation:

• Sort a list of integers, from low to high
• Split a list of integers into 2 parts (by a middle) and put them into 2 new separate ArrayLists (call them "left" and "right")
• Find a middle number (median) in both of those new ArrayLists
• Q1 is a median from left side, and Q3 is the median from the right side
• Applying mathematical formula:
• IQR = Q3 - Q1
• LowerFence = Q1 - 1.5*IQR
• UpperFence = Q3 + 1.5*IQR
• More info about this formula: http://www.mathwords.com/o/outlier.htm
• Loop through all of my original elements, and if any of them are lower than a lower fence, or higher than an upper fence, add them to "output" ArrayList
• This new "output" ArrayList contains the outliers