```public class SD
{

/**
* Calculates the sample standard deviation of an array
* of numbers, when the number are obtained by a random sampling.
* Code must be modified if you have a complete set of data.
* To get estimate of a complete data sampling, use n instead of n-1 in last line.
*
* see http://davidmlane.com/hyperstat/A16252.html
*
* @param data Numbers to compute the standard deviation of.
* Array must contain two or more numbers.
* @return standard deviation estimate of population
*/
public static double sdFast ( double[] data )
{
// sd is sqrt of sum of (values-mean) squared divided by n - 1
// Calculate the mean
double mean = 0;
final int n = data.length;
if ( n < 2 )
{
return Double.NaN;
}
for ( int i=0; i<n; i++ )
{
mean += data[i];
}
mean /= n;
// calculate the sum of squares
double sum = 0;
for ( int i=0; i<n; i++ )
{
final double v = data[i] - mean;
sum += v * v;
}
// Change to ( n - 1 ) to n if you have complete data instead of a sample.
return Math.sqrt( sum / ( n - 1 ) );
}

/**
* Calculates the sample standard deviation of an array
* of numbers,  when the number are obtained by a random sampling.
* Code must be modified if you have a complete set of data.
* To get estimate of a complete sample, use n instead of n-1 in last line.
*
* see Knuth's The Art Of Computer Programming
* Volume II: Seminumerical Algorithms
* This algorithm is slower, but more resistant to error propagation.
*
* @param data Numbers to compute the standard deviation of.
* Array must contain two or more numbers.
* @return standard deviation estimate of population
*/
public static double sdKnuth ( double[] data )
{
final int n = data.length;
if ( n < 2 )
{
return Double.NaN;
}
double avg = data;
double sum = 0;
for ( int i = 1; i < data.length; i++ )
{
double newavg = avg + ( data[i] - avg ) / ( i + 1 );
sum += ( data[i] - avg ) * ( data [i] -newavg ) ;
avg = newavg;
}
// Change to ( n - 1 ) to n if you have complete data instead of a sample.
return Math.sqrt( sum / ( n - 1 ) );
}

/**
* Test driver
*
* @param args not used
*/
public static void main ( String[] args )
{
double[] data = { 10, 100 , 50};
out.println( sdFast( data ) );
out.println( sdKnuth( data ) );
}
}```