class to compute the running average of numbers passed to it
**/
public class Averager {
// Private fields to hold the current state.
private int n = 0;
private double sum = 0.0, sumOfSquares = 0.0;

/**
* This method adds a new datum into the average.
**/
public void addDatum(double x) {
n++;
sum += x;
sumOfSquares += x * x;
}

/** This method returns the average of all numbers passed to addDatum() */
public double getAverage() { return sum / n; }

/** This method returns the standard deviation of the data */
public double getStandardDeviation() {
return Math.sqrt(((sumOfSquares - sum*sum/n)/n));
}

/** This method returns the number of numbers passed to addDatum() */
public double getNum() { return n; }

/** This method returns the sum of all numbers passed to addDatum() */
public double getSum() { return sum; }

/** This method returns the sum of the squares of all numbers. */
public double getSumOfSquares() { return sumOfSquares; }

/** This method resets the Averager object to begin from scratch */
public void reset() { n = 0; sum = 0.0; sumOfSquares = 0.0; }

/**
* This nested class is a simple test program we can use to check that
* our code works okay.
**/
public static class Test {
public void work(){
Averager a = new Averager();
for(int i = 1; i <= 100; i++) a.addDatum(i);
System.out.println("Average: " + a.getAverage());
System.out.println("Standard Deviation: " + a.getStandardDeviation());
System.out.println("N: " + a.getNum());
System.out.println("Sum: " + a.getSum());
System.out.println("Sum of squares: " + a.getSumOfSquares());
}
}
}

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