numpy的nonzero函数
看官方文档：
In [10]: help(np.nonzero)
Help on function nonzero in module numpy.core.fromnumeric:
nonzero(a)
Return the indices of the elements that are non-zero.
Returns a tuple of arrays, one for each dimension of `a`, containing
the indices of the non-zero elements in that dimension. The
corresponding non-zero values can be obtained with::
a[nonzero(a)]
To group the indices by element, rather than dimension, use::
transpose(nonzero(a))
The result of this is always a 2-D array, with a row for
each non-zero element.
Parameters
----------
a : array_like
Input array.
Returns
-------
tuple_of_arrays : tuple
Indices of elements that are non-zero.
See Also
--------
flatnonzero :
Return indices that are non-zero in the flattened version of the input
array.
ndarray.nonzero :
Equivalent ndarray method.
count_nonzero :
Counts the number of non-zero elements in the input array.
Examples
--------
>>> x = np.eye(3)
>>> x
array([[ 1., 0., 0.],
[ 0., 1., 0.],
[ 0., 0., 1.]])
>>> np.nonzero(x)
(array([0, 1, 2]), array([0, 1, 2]))
>>> x[np.nonzero(x)]
array([ 1., 1., 1.])
>>> np.transpose(np.nonzero(x))
array([[0, 0],
[1, 1],
[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, np.nonzero(a > 3)
yields the indices of the `a` where the condition is true.
>>> a = np.array([[1,2,3],[4,5,6],[7,8,9]])
>>> a > 3
array([[False, False, False],
[ True, True, True],
[ True, True, True]], dtype=bool)
>>> np.nonzero(a > 3)
(array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))
The ``nonzero`` method of the boolean array can also be called.
>>> (a > 3).nonzero()
(array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2])) #坐标(1,0)(1,1)(1,2)(2,0)(2,1)(2,2)非零
numpy函数返回非零元素的目录。
返回值为元组， 两个值分别为两个维度， 包含了相应维度上非零元素的目录值。 可以通过a[nonzero(a)]来获得所有非零值。
nonzero(a) 将对矩阵a的所有非零元素， 分别安装两个维度， 一次返回其在各维度上的目录值。
如果 a=mat([ [1,0,0],
[0,0,0],
[0,0,0]])
则 nonzero(a) 返回值为 (array([0]), array([0])) , 因为矩阵a只有一个非零值， 在第0行， 第0列。
如果 a=mat([ [1,0,0],
[1,0,0],
[0,0,0]])
则 nonzero(a) 返回值为 (array([0, 1]), array([0, 0])) , 因为矩阵a只有两个非零值， 在第0行、第0列，和第1行、第0列。所以结果元组中，第一个行维度数据为（0,1） 元组第二个列维度都为（0,0）。
