numpy.ma.MaskedArray.nonzero¶
method

MaskedArray.
nonzero
()[source]¶ Return the indices of unmasked elements that are not zero.
Returns a tuple of arrays, one for each dimension, containing the indices of the nonzero elements in that dimension. The corresponding nonzero 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 nonzero element.
Parameters:  None
Returns:  tuple_of_arrays : tuple
Indices of elements that are nonzero.
See also
numpy.nonzero
 Function operating on ndarrays.
flatnonzero
 Return indices that are nonzero in the flattened version of the input array.
ndarray.nonzero
 Equivalent ndarray method.
count_nonzero
 Counts the number of nonzero elements in the input array.
Examples
>>> 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=999999) >>> 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]))