切片3D Numpy数组-误解

Question

I know for 3d numpy array I can index like:

item = x[0,2,1] 

or

item = x[0][2][1]

But slicing work strange for me:

item = x[:,:,1]

is not the same as:

item = x[:][:][1]

What did I miss?

Answer 1

x[:] will return the full array, without doing any actual slicing. By that logic, so will x[:][:] .

As such, x[:][:][1] is equivalent to x[1] . This is why it's not the same as x[:,:,1] .

Answer 2

I like @ffisegydd's answer, but I wanted to point out that this is not unique to numpy arrays. In python the statement result = A[i, j] is equivalent to result = A[(i, j)] and the statement result = A[i][j] is equivalent to:

tmp = A[i]
result = tmp[j]

So if I use a dictionary:

A = {0 : "value for key 0",
     (0, 1) : "value for key (0, 1)"}
print(A[0][1])
# a
print(A[0, 1])
# value for key (0, 1)