Counting in Array List/ Standard deviation

[Ron Ingram]

Hello all,

I'm working on an assignment that I can't seem to get my arms around. Basically, I have to supply values and count the number of entries, provide a sum for each number added,an average and standard deviation. Right now, I'm trying to correctly add each entry into the list and get the count, sum and average. For some reason, it doubles each entry within the LIst, so that if I provide one entry of '5', it adds it twice when getting the sum.

Please review what I have and provide any/all the help you can. Thank you.

This is the tester class:

public class DataSetTester
{
   public static void main(String[] args)
   {
      DataSet a = new DataSet();
      a.add(5);
      a.add(6);
      a.add(8);
      a.add(9);
      System.out.println("count: " + a.getCount());
      System.out.println("Expected: 4");
      System.out.println("average: " + a.getAverage());
      System.out.println("Expected: 7");
      //System.out.println("standard deviation: " + a.getStandardDeviation());
      System.out.println("Expected: 1.83");
   }
}

Here is the dataset class

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

/**
 *
 * @author jettaman757
 */
/**
This class is used to calculate the average and standard deviation
of a data set.
 */
public class DataSet {

    private double sum;
    private double sumSquare;
    private int n;
    private double avg;
    private List<Double> inputList = new ArrayList();

    /**
    Constructs a DataSet object to hold the total number of inputs,
    the sum, and the sum of square.
     */
    public DataSet() {
    }

    /**
    Adds a value to this data set.
    @param x the input value
     */
    public void add(double x) {

        n++;
        sum = sum + x;
        sumSquare = sumSquare + x * x;



        inputList.add(x);
        System.out.println("Array list size is: "+inputList.size());
        

        for (int i=0; i < inputList.size(); i++){

            
            System.out.println("Values: "+inputList.get(i));
            sum += x;
            System.out.println("Sum = "+sum);






        }






    }

    /**
    Method used to calculate the average of the data set.
    @return average the average of the data set
     */
    public double getAverage() {
 

        avg = sum / n;
        System.out.println("Avg is: " + avg);

        return avg;

    }

    /**
    Method used to calculate the standard deviation.
    @return stddev the standard deviation of data set
     */
    public double getStandardDeviation(double[] x) {


        if (x == null) {
            throw new IllegalArgumentException("null array");
        }

        double dev;

        for (int i = 0; i < x.length; i++) {
            sum += Math.pow((x[i] - avg), 2);

        }

        dev = Math.sqrt(sum / (n)); // maybe n should go here?




        System.out.println(" The standard deviation is");
        return dev;
        /*
        double stddev = Math.sqrt(sum / n - avg * avg);

        return stddev;
         *
         */

    }

    /**
    Gets the total number of input values.
    @return n the total number of inputs
     */
    public int getCount() {
        return n;

    }
}

[Ove Lindström]

Yep.

On row 34 you add the new number to the sum.

On row 43 you start to loop and add the new number again.

Why not make a method that sums it up when asked to do it? One method should do one thing. Add-method should add the number to the list, sum-method return the sum. avg-method the average.

[Ron Ingram]

Hello,

Yes, I'm working towards cleaning it up, but I'm really stuck on the array. I've corrected and here is the new Dataset class and output. Seems to still be doubling in the add() method.

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

/**
 *
 * @author jettaman757
 */
/**
This class is used to calculate the average and standard deviation
of a data set.
 */
public class DataSet {

    private double sum;
    private double sumSquare;
    private int n;
    private double avg;
    private List<Double> inputList = new ArrayList();

    /**
    Constructs a DataSet object to hold the total number of inputs,
    the sum, and the sum of square.
     */
    public DataSet() {
    }

    /**
    Adds a value to this data set.
    @param x the input value
     */
    public void add(double x) {

        n++;
        //sum = sum + x;
        //sumSquare = sumSquare + x * x;



        inputList.add(x);
        System.out.println("Array list size is: "+inputList.size());
        

        for (int i=0; i < inputList.size(); i++){

            
            System.out.println("Values: "+inputList.get(i));
            sum += x;
            System.out.println("Sum = "+sum);






        }






    }

    /**
    Method used to calculate the average of the data set.
    @return average the average of the data set
     */
    public double getAverage() {
 

        avg = sum / n;
        System.out.println("Avg is: " + avg);

        return avg;

    }

    /**
    Method used to calculate the standard deviation.
    @return stddev the standard deviation of data set
     */
    public double getStandardDeviation(double[] x) {


        if (x == null) {
            throw new IllegalArgumentException("null array");
        }

        double dev;

        for (int i = 0; i < x.length; i++) {
            sum += Math.pow((x[i] - avg), 2);

        }

        dev = Math.sqrt(sum / (n)); // maybe n should go here?




        System.out.println(" The standard deviation is");
        return dev;
        /*
        double stddev = Math.sqrt(sum / n - avg * avg);

        return stddev;
         *
         */

    }

    /**
    Gets the total number of input values.
    @return n the total number of inputs
     */
    public int getCount() {
        return n;

    }
}

Output:

Array list size is: 1
Values: 5.0
Sum = 5.0
Array list size is: 2
Values: 5.0
Sum = 11.0
Values: 6.0
Sum = 17.0
Array list size is: 3
Values: 5.0
Sum = 25.0
Values: 6.0
Sum = 33.0
Values: 8.0
Sum = 41.0
Array list size is: 4
Values: 5.0
Sum = 50.0
Values: 6.0
Sum = 59.0
Values: 8.0
Sum = 68.0
Values: 9.0
Sum = 77.0
count: 4
Expected: 4
Avg is: 19.25
average: 19.25
Expected: 7
Expected: 1.83
BUILD SUCCESSFUL (total time: 0 seconds)

[Ove Lindström]

Ask yourself: Why do I need to loop through all the numbers already added in the list and for every number found there, add x to the sum?

I've done something like this:

package lilly;

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

/** 
 * 
 * @author jettaman757 
 */
/**
 * This class is used to calculate the average and standard deviation of a data
 * set.
 */
public class DataSet {

	private List<Double> inputList = new ArrayList();

	/**
	 * Constructs a DataSet object to hold the total number of inputs, the sum,
	 * and the sum of square.
	 */
	public DataSet() {
	}

	/**
	 * Adds a value to this data set.
	 * 
	 * @param x
	 *            the input value
	 */
	public void add(double x) {

		inputList.add(x);
		System.out.println("Array list size is: " + inputList.size());
	}

	/**
	 * Method used to calculate the average of the data set.
	 * 
	 * @return average the average of the data set
	 */
	public double getAverage() {

		double sum = getSum();
		double count = getCount();
		double avg = sum / count;
		System.out.println("Avg is: " + avg);
		return avg;

	}

	public double getSum() {
		double sum = 0d;
		
		for (double d : inputList) {
			sum += d;
		}
		return sum;
	}

	/**
	 * Gets the total number of input values.
	 * 
	 * @return n the total number of inputs
	 */
	public int getCount() {
		return inputList.size();

	}
	
	public static void main(String[] args)  
	   {  
	      DataSet a = new DataSet();  
	      a.add(5);  
	      a.add(6);  
	      a.add(8);  
	      a.add(9);  
	      System.out.println("count: " + a.getCount());  
	      System.out.println("Expected: 4");  
	      System.out.println("average: " + a.getAverage());  
	      System.out.println("Expected: 7");  
	      //System.out.println("standard deviation: " + a.getStandardDeviation());  
	      //.out.println("Expected: 1.83");  
	   }  
}

[Ron Ingram]

Good point..lol. Thanks for the help tonight. Ron

[Ron Ingram]

To follow up in case someone is left working the same problem, I've completed this program using the following Dataset class:

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

/**
 *
 * @author jettaman757
 */
/**
 * This class is used to calculate the average and standard deviation of a data
 * set.
 */
public class DataSet {

    private List<Double> inputList = new ArrayList();
    double x = 0;

    /**
     * Constructs a DataSet object to hold the total number of inputs, the sum,
     * and the sum of square.
     */
    public DataSet() {
    }

    /**
     * Adds a value to this data set.
     *
     * @param x
     *            the input value
     */
    public void add(double x) {

        inputList.add(x);
        System.out.println("Array list size is: " + inputList.size());
    }

    /**
     * Method used to calculate the average of the data set.
     *
     * @return average the average of the data set
     */
    public double getAverage() {

        double sum = getSum();
        double count = getCount();
        double avg = sum / count;
        System.out.println("Avg is: " + avg);
        return avg;

    }

    public double getSum() {
        double sum = 0d;

        for (double d : inputList) {
            sum += d;
        }
        return sum;
    }

    public double getStandardDeviation() {

        /*
         * To find the Standard Deviation, Follow 6 Steps:
         *
         * STEP 1:  Find the average or mean (Total / count)
         * STEP 2:  Differences-- Subtract the avg/mean from each value that was added
         * STEP 3:  Square root each remainder from STEP 2
         * STEP 4:  Total the all remainders from STEP 3
         * STEP 5:  Divide total from STEP 4 by one less than the count of all numbers
         * STEP 6:  Square root the value from STEP 5 = Standard Deviation
         */

        double sum = getSum();
        //STEP 1
        double avg = getAverage();
        double calc1 = 0;
        double calc2 = 0;
        double count = getCount();
        double stdDeviation = 0;

        System.out.println("Sum = " + sum);

        /*STEP 2:  Calculate the subtraction of Avg/Mean from each value
        Create an iterator in order to access(get) the values in inputList
         */
        for (int i = 0; i < count; i++) {
            calc1 = inputList.get(i) - avg;
            System.out.println("Calc1= " + calc1);

        // STEP 3:  Square root each remainder from STEP 2

            calc1 = Math.pow(calc1, 2);
            System.out.println("STEP 3 calc= " + calc1);

        // STEP 4:  Total the all remainders from STEP 3
            calc2 = calc2 + calc1;
            // Add calc2 into a new array for further calculations

            System.out.println("STEP 4 calc = " + calc2);
        }

        // STEP 5:  Divide total from STEP 4 by one less than the count of all numbers
            calc2 = calc2 / (count-1);
            System.out.println("STEP 5 calc= "+calc2);

        // STEP 6:  Square root the value from STEP 5 = Standard Deviation
            stdDeviation = Math.sqrt(calc2);

        // Return Standard Deviation Value
        return stdDeviation;
    }

    /**
     * Gets the total number of input values.
     *
     * @return n the total number of inputs
     */
    public int getCount() {
        return inputList.size();

    }
}