Finding the standard deviation from a 2D Array

Question 1

u need 2 loops since u need to subtract average from every row.

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);

Question 2

Here's how I'd do it.

You can manage it without an array. Just keep track of the sum of values, sum of values squared, and number of values.

I only added List of grades for display purposes. Not part of mean or variance calculations.

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;
    }
}

Question 3

I just would like to note that to estimate the standard deviation from a sample, one normally uses the equation

stdDev=Math.sqrt(sum/N-1);

where sum is the sum over all quadratic deviations and N is the sample size. So in your example, you would have to divide by 9, not 10. Maybe this produces the numbers you expect.

(see http://en.wikipedia.org/wiki/Unbiased_estimation_of_standard_deviation)