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# numpy.ma.MaskedArray.nonzero

method

ma.MaskedArray.nonzero()

  Return the indices of unmasked elements that are not zero.

  Returns a tuple of arrays, one for each dimension, containing the indices of the non-zero elements in that dimension. The corresponding non-zero values can be obtained with:

      a[a.nonzero()]

  To group the indices by element, rather than dimension, use instead:

      np.transpose(a.nonzero())

  The result of this is always a 2d array, with a row for each non-zero element.

  Parameters:
    None
  Returns:
    tuple_of_arrays tuple

      Indices of elements that are non-zero.

  See also

  `numpy.nonzero`

    Function operating on ndarrays.

  `flatnonzero`

    Return indices that are non-zero in the flattened version of the input array.

  `numpy.ndarray.nonzero`

    Equivalent ndarray method.

  `count_nonzero`

    Counts the number of non-zero elements in the input array.

  Examples

      >>> import numpy as np
      >>> import numpy.ma as ma
      >>> x = ma.array(np.eye(3))
      >>> x
      masked_array(
        data=[[1., 0., 0.],
              [0., 1., 0.],
              [0., 0., 1.]],
        mask=False,
        fill_value=1e+20)
      >>> x.nonzero()
      (array([0, 1, 2]), array([0, 1, 2]))

  Masked elements are ignored.

      >>> x[1, 1] = ma.masked
      >>> x
      masked_array(
        data=[[1.0, 0.0, 0.0],
              [0.0, --, 0.0],
              [0.0, 0.0, 1.0]],
        mask=[[False, False, False],
              [False,  True, False],
              [False, False, False]],
        fill_value=1e+20)
      >>> x.nonzero()
      (array([0, 2]), array([0, 2]))

  Indices can also be grouped by element.

      >>> np.transpose(x.nonzero())
      array([[0, 0],
             [2, 2]])

  A common use for `nonzero` is to find the indices of an array, where a condition is True. Given an array a, the condition a > 3 is a boolean array and since False is interpreted as 0, ma.nonzero(a > 3) yields the indices of the a where the condition is true.

      >>> a = ma.array([[1,2,3],[4,5,6],[7,8,9]])
      >>> a > 3
      masked_array(
        data=[[False, False, False],
              [ True,  True,  True],
              [ True,  True,  True]],
        mask=False,
        fill_value=True)
      >>> ma.nonzero(a > 3)
      (array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))

  The `nonzero` method of the condition array can also be called.

      >>> (a > 3).nonzero()
      (array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))