ArrayList的标准偏差

Question

该代码应该找到一个随机整数的标准偏差ArrayList.但是,我的标准偏差代码没有显示正确的结果.它显示了预期的另一个数字.

我究竟做错了什么？

import java.io.*;
import java.util.*;

public class Assignment4 {
    public static void main(String[] args)
    {
        ArrayList<Integer> values = new ArrayList<Integer>();
        int count = 0;
        int total = 0;
        Random r = new Random();

        for (int i = 1; i <= 10; i++) {
            values.add(r.nextInt(90)+ 1);

            System.out.println(values);

        }

        System.out.println(mean(values));
        System.out.println(sd(values));
    }

    public static double mean (ArrayList<Integer> table)
    {
        int total = 0;

        for ( int i= 0;i < table.size(); i++)
        {
            int currentNum = table.get(i);
            total+= currentNum;
        }
        return total/table.size();
    }

    public static double sd (ArrayList<Integer> table)
    {
        double mean= mean(table);
        double temp =0;
        for ( int i= 0; i <table.size(); i++)
        {
            temp= Math.pow(i-mean, 2);
        }

        return Math.sqrt(mean( table));
    }

    public static void selectionSort(ArrayList<Integer> table)
    {
        int count = table.size();
        for(int pos = 0; pos < count - 1; pos++)
        {
            int locMin = pos;
            for(int i = pos + 1; i < count; i++)
            {
                if(table.get(i) < table.get(locMin))
                    locMin = i;
            }

            int temp = table.get(pos);
            table.set(pos, table.get(locMin) );
            table.set(locMin, temp);
        }
    }
}

Answer 1

您的标准偏差计算有错误:

这是查找标准差的算法:

Step 1: Find the mean.
Step 2: For each data point, find the square of its distance to the mean.
Step 3: Sum the values from Step 2.
Step 4: Divide by the number of data points.
Step 5: Take the square root.

因此,您的代码应如下所示:

public static double sd (ArrayList<Integer> table)
{
    // Step 1: 
    double mean = mean(table);
    double temp = 0;

    for (int i = 0; i < table.size(); i++)
    {
        int val = table.get(i);

        // Step 2:
        double squrDiffToMean = Math.pow(val - mean, 2);

        // Step 3:
        temp += squrDiffToMean;
    }

    // Step 4:
    double meanOfDiffs = (double) temp / (double) (table.size());

    // Step 5:
    return Math.sqrt(meanOfDiffs);
}

注意:您的平均值计算精度会有所下降.

你有:

return total/table.size();

它应该是:

return (double) total / (double) table.size();