numpy找非零元素并计数 numpy.nonzero 和 numpy.count_nonzero

numpy.nonzero(a)
Return the indices of the elements that are non-zero.
示例

x = np.array([[3, 0, 0], [0, 4, 0], [5, 6, 0]])
np.nonzero(x)

(array([0, 1, 2, 2], dtype=int64), array([0, 1, 0, 1], dtype=int64))

也就是 [0 0], [1 1], [2 0], [2 1]位置的元素不为0

numpy.count_nonzero(a, axis=None)
Counts the number of non-zero values in the array a.

np.count_nonzero(np.eye(4))

4

np.count_nonzero([[0,1,7,0,0],[3,0,0,2,19]], axis=0)

array([1, 1, 1, 1, 1])

np.count_nonzero([[0,1,7,0,0],[3,0,0,2,19]], axis=1)

array([2, 3])