code: public static void standardize(double[] x) {
          double mu = mean(x);
          double sigma = sd(x);
  
          if (isZero(sigma)) {
              logger.warn("array has variance of 0.");
              return;
          }
  
          for (int i = 0; i < x.length; i++) {
              x[i] = (x[i] - mu) / sigma;
          }
      }
docs: Standardizes an array to mean 0 and variance 1.