public double std(){
  if (min == max || num == 0) {
    return 0;
  }
  double m=mean();
  return Math.sqrt(sigmax2 / num - m * m);
}

public double std ( ) { if ( min == max || num == 0 ) { return 0 ; } double m = mean ( ) ; return Math . sqrt ( sigmax2 / num - m * m ) ; }
calculate the population standard deviation . $ \ sqrt ( \ frac { \ sum ( x - m ) ^ 2 } { n } ) $
