code: def center(self, X):
          
          X = X.copy()
          inan = numpy.isnan(X)
          if self.mu is None:
              X_ = numpy.ma.masked_array(X, inan)
              self.mu = X_.mean(0).base
              self.sigma = X_.std(0).base
          reduce(lambda y,x: setitem(x[0], x[1], x[2]), zip(X.T, inan.T, self.mu), None)
          X = X - self.mu
          X = X / numpy.where(self.sigma == 0, 1e-30, self.sigma)
          return X
docs: Center `X` in PCA space.