code: def _sparseness(x):
      """Hoyer's measure of sparsity for a vector"""
      sqrt_n = np.sqrt(len(x))
      return (sqrt_n - np.linalg.norm(x, 1) / norm(x)) / (sqrt_n - 1)
  
imports: from __future__ import division, print_function
  from math import sqrt
  import warnings
  import numbers
  import numpy
  import scipy.sparse
  from ..base import BaseEstimator, TransformerMixin
  from ..utils import check_random_state, check_array
  from ..utils.extmath import randomized_svd, safe_sparse_dot, squared_norm
  from ..utils.extmath import fast_dot
  from ..utils.validation import check_is_fitted, check_non_negative
  from ..utils import deprecated
  from ..exceptions import ConvergenceWarning
  from .cdnmf_fast import _update_cdnmf_fast
label_desc: Use image node_id 4 for calling a global function with example usage: _sparseness(x) and returns: unknown