code: public static double stdDev(double[] a) {
  
      	if(a == null || a.length == 0)
      		return -1;
      	
          return Math.sqrt(varp(a));
      }
docs: Returns the population standard deviation in the specified array.
  
  @param  a the array
  @return the population standard deviation in the array;
  {@code Double.NaN} if no such value