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

See also
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]))