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Calculating Variance

Posted on 2006-11-22
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Last Modified: 2010-05-12
HI,
I have to calculate the Variance of a user input in Java. I have to use this formula:
V(X) = E(X²) - (E(X))²

The user input may consist of an arbitrary amount of integers from 1 to 5.

The thing is that I have absolutely no clue how to code this formula. So if anyone can help me it would be really great.
The best solution for me would be a code snipplet which calculates the variance.

Nevertheless if you can tell me how the formula really works and what the variables used in the code mean.
This would be definitely the best thing.

thank, michi
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Question by:michi3012
  • 2
4 Comments
 
LVL 25

Expert Comment

by:InteractiveMind
ID: 17998991
The variance is the mean of the square, minus the square of the mean.

So if you have a data set (in this case, X), then you simply square each number, and find the mean (sum and divide by how many there are), then you subtract from this the mean of the set squared.


public double Var(int [] X)
{
    double E_X2=0.0;
    double EX_2=0.0;
    for(int i=0; i<X.length; i++)
    {
        E_X2 += X[i]*X[i];
        EX_2 += X[i];
    }
    E_X2 /= X.length;
    EX_2 /= X.length;
   
    return E_X2 - EX_2;
}
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LVL 25

Accepted Solution

by:
InteractiveMind earned 2000 total points
ID: 17998999
Sorry

public double Var(int [] X)
{
    double E_X2=0.0;
    double EX_2=0.0;
    for(int i=0; i<X.length; i++)
    {
        E_X2 += X[i]*X[i];
        EX_2 += X[i];
    }
    E_X2 /= X.length;
    EX_2 /= X.length;
   
    return E_X2 - EX_2*EX_2;
}
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LVL 2

Expert Comment

by:saintsairforce
ID: 17999070
This should help

public void variance()
{
     Vector<int> numbers = new Vector<int>();
     
     //file your vector
//note a vector is like an array by the size can grow easily.

     double sumX = sumX(numbers);
     double variance = sumXSquared(numbers)+sumx*sumX;

//display your variance value
}


public double xSquared(Vector<int> numbers)
{
   double total=0;
  for(int cnt=0;cnt<numbers.size();cnt++)
  {
     total += numbers.get(cnt)*numbers.get(cnt);
  }

  return total;
}

public double sumX(Vector<int> numbers)
{
     double total=0;
    for(int cnt=0;cnt<numbers.size();cnt++)
  {
     total += numbers.get(cnt);
  }

  return total;
}

cheers,
Ricky
0
 
LVL 10

Expert Comment

by:ADSLMark
ID: 17999082
Hi,

V(X) = variance.
E(X) = expected value.
Wikipedia has more info: http://en.wikipedia.org/wiki/Variance

Here is some code.

class VarianceExample
{
    public static void main(String[] args)
    {
        double ex2 = 0;
        double e2x = 0;
        double ex = 0;
        double vx = 0;

        int[] xs = new int[]{1, 4, 2, 3};
        int numxs = xs.length;

        for(int i=0;i<numxs;i++)
        {
            ex2 += xs[i] * xs[i];
            ex += xs[i];
        }
        ex2 /= numxs;   //E(X^2)
        ex /= numxs;    //E(X)
        e2x = ex * ex;  //E(X)^2
        vx = ex2 - e2x; //V(X)

        //String representation of xs
        String sxs = "";
        for(int i=0;i<numxs-1;i++)
            sxs += xs[i]+", ";
        sxs += xs[numxs-1];

        System.out.println("X      = "+sxs);
        System.out.println("E(X)   = "+ex);
        System.out.println("E(X^2) = "+ex2);
        System.out.println("E(X)^2 = "+e2x);
        System.out.println("V(X)   = "+vx);
    }
}

Mark
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