我正在努力为我的任务找到标准偏差，我已经做了其他所有事情，而我只是在最后一点上苦苦挣扎。我的作业问题简而言之是： 我收到了一组学生的作业结果的文本文件（必须是任何数字）。给出的唯一数据是学生姓名，粉丝及其结果，您需要计算他们的平均成绩和标准差。输出应如下所示：
Student Name FAN Part 1 Part 2 Part 3 Part 4 Mark Grade
Adam Adamson adam0001 85.4 79.8 82.4 86.1 82.77% DN
Bethany Bright brig0001 89.7 85.6 84.2 82.9 84.92% DN
Cameron Carlson carl0001 55.45 49.82 60.4 42.27 50.23% P
David Dawson daws0001 72.6 78.49 80.2 65.88 74.46% CR
Evelyn Ellis elli0001 50.2 35.88 48.41 58.37 46.57% FA
Frances Fitz fitz0001 78.9 75.67 82.48 79.1 78.38% DN
Greg Gregson greg0001 24.3 32.88 29.72 28.4 30.05% F
Harriett Hope hope0001 52.2 58.93 61.5 63.44 60.12% P
Ivan Indigo indi0001 88.4 91.23 90.05 92.46 91.08% HD
Jessica Jones jone0001 82.33 89.74 81.3 84.85 85.84% HD
Average 67.948 67.804 70.066 68.377 68.44% CR
StdDev 19.4442
我知道我需要使用的等式，我知道我需要使用哪些数字我只是不知道如何获得它们。我会告诉你我需要做什么，希望你能进一步了解我的代码：
the total of the averages divided by 10 = 68.442
sum = (82.77-68.442)^2 + (84.92-68.442)^2 + (50.23-68.442)^2 + ... + (85.84-68.442)^2
StdDev = Math.sqrt(sum/10)
stdDev = 19.4442
我正在使用3个课程，我的主要课程Topic Management，一个Student课程，其中包含一个带有学生姓名和粉丝的数组（两者都与我的问题无关，但我认为我最好设置场景） ，最后是StudentsMarks Class，其中包含我的StdDec方法，如下所示：
public static double StdDev() throws IOException
{
String [][] marks = StudentMarks.StudentMarks();
double mean = 0, average = 0, stdDev = 0;
int row = 0;
for(row = 0; row < marks.length; row++)
{ //ROW,COL
double score1 = Double.parseDouble(marks[row][2]); //parsing the data from the array into a double
double score2 = Double.parseDouble(marks[row][3]);
double score3 = Double.parseDouble(marks[row][4]);
double score4 = Double.parseDouble(marks[row][5]);
average = score1*0.1 + score2*0.4 + score3*0.2 + score4*0.3;
mean = average/row;
stdDev = Math.sqrt(Math.pow(average - mean,2)/row);
}
return stdDev;
}
答案 0 :(得分：1)
你需要2个循环，因为你需要从每一行中减去平均值。
double[] sum = new double[marks.length];
for(row = 0; row < marks.length; row++)
{ //ROW,COL
double score1 = Double.parseDouble(marks[row][2]); //parsing the data from the array into a double
double score2 = Double.parseDouble(marks[row][3]);
double score3 = Double.parseDouble(marks[row][4]);
double score4 = Double.parseDouble(marks[row][5]);
sum[row] = score1*0.1 + score2*0.4 + score3*0.2 + score4*0.3;
average += sum[row];
}
average = average/sum.length; //1
for(row = 0; row < sum.length; row++)
{
mean += Math.pow(Math.abs(sum[row] - average),2);
}
mean = mean/marks.length;//2
stdDev = Math.sqrt(mean);
答案 1 :(得分：0)
以下是我的表现。
您可以在没有数组的情况下进行管理。只需跟踪值的总和，平方值的平方和值的数量。
我只为展示目的添加了成绩列表。不属于均值或方差计算的一部分。
package reportcard;
import org.apache.commons.lang3.StringUtils;
/**
* Student
* @author Michael
* @link http://stackoverflow.com/questions/19077442/finding-the-standard-deviation-from-a-2d-array
* @since 9/29/13 12:19 PM
*/
public class Student {
private final String name;
public Student(String name) {
if (StringUtils.isBlank(name)) throw new IllegalArgumentException("Name cannot be blank or null");
this.name = name;
}
public String getName() {
return name;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Student student = (Student) o;
if (!name.equals(student.name)) return false;
return true;
}
@Override
public int hashCode() {
return name.hashCode();
}
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("Student{");
sb.append("name='").append(name).append('\'');
sb.append('}');
return sb.toString();
}
}
package reportcard;
import java.util.ArrayList;
import java.util.List;
/**
* Transcript
* @author Michael
* @link http://stackoverflow.com/questions/19077442/finding-the-standard-deviation-from-a-2d-array
* @since 9/29/13 12:21 PM
*/
public class Transcript {
private final Student student;
private final List<Double> grades;
private double sum = 0.0;
private double sumsq = 0.0;
private int numGrades = 0;
public static void main(String[] args) {
Student student = new Student("Fatty Arbuckle");
Transcript transcript = new Transcript(student);
double [] grades = { 2, 4, 4, 4, 5, 5, 7, 9 };
for (double grade : grades) {
transcript.addGrade(grade);
}
System.out.println(transcript);
}
public Transcript(Student student) {
if (student == null) throw new IllegalArgumentException("Student cannot be null");
this.student = student;
this.grades = new ArrayList<Double>();
}
public void addGrade(double grade) {
if (grade > 0.0) {
this.grades.add(grade);
++this.numGrades;
this.sum += grade;
this.sumsq += grade*grade;
}
}
public double getMean() {
double mean = 0.0;
if (this.numGrades > 0) {
mean = this.sum/this.numGrades;
}
return mean;
}
public double getVariance() {
double stddev = 0.0;
if (this.numGrades > 0) {
stddev = Math.sqrt((this.sumsq-this.sum*this.sum/this.numGrades)/this.numGrades);
}
return stddev;
}
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("Transcript{");
sb.append("student=").append(student);
sb.append(", grades=").append(grades);
sb.append(", sum=").append(sum);
sb.append(", sumsq=").append(sumsq);
sb.append(", numGrades=").append(numGrades);
sb.append(", mean=").append(this.getMean());
sb.append(", variance=").append(this.getVariance());
sb.append('}');
return sb.toString();
}
}
package reportcard;
/**
* Grade encapsulates grades with values
* @author Michael
* @link http://stackoverflow.com/questions/16744116/java-grade-class-letter-grade-to-number/16744210#16744210
* @since 5/24/13 6:01 PM
*/
public enum Grade {
APLUS("A+", 4.3),
A("A", 4.0),
AMINUS("A-", 3.7),
BPLUS("B+", 3.3),
B("B", 3.0),
BMINUS("B-", 2.7),
CPLUS("C+", 2.3),
C("C", 2.0),
CMINUS("C-", 1.7),
DPLUS("D+", 1.3),
D("D", 1.0),
DMINUS("D-", 0.7),
F("F", 0.0);
private final String symbol;
private final double value;
Grade(String symbol, double value) {
this.symbol = symbol;
this.value = value;
}
public String getSymbol() {
return symbol;
}
public double getValue() {
return value;
}
}
答案 2 :(得分：0)
我只想指出，为了估计样本的标准偏差，通常使用等式
stdDev=Math.sqrt(sum/N-1);
其中sum是所有二次偏差的总和，N是样本大小。所以在你的例子中，你必须除以9，而不是10.也许这会产生你期望的数字。
（见http://en.wikipedia.org/wiki/Unbiased_estimation_of_standard_deviation）