在切片二维 python numpy 数组中检索 null 值

Question

I need to slice a python numpy array for multiplication. The slicing I need to be in the below format

    first_slice_matrixA=[0,1]
    second_slice_matriA=[2,3]
    first_slice_matrixB=[0,2]
    second_slice_matrixB=[1,3]

Below is my code for slicing. For the first slice its working but for second it returns a null array.

   matrix_A = np.arange(4).reshape(2,2)
   matrix_B=np.arange(4).reshape(2,2)
   A_1=matrix_A[0:1, 0:2]
   A_2=matrix_A[1:1, 0:2]
   print(A_2)

Thank you for your help

Answer 1

When you use same index for start and stop, the slice is empty, whatever the index is. You'd do the following so see your values

• first index is vertical slicing, so it selects 'rows'
• second index is horizontal slicing, so it selects 'columns'

m = np.arange(4).reshape(2,2)

# horizontal
print(m[0:1, :]) # [[0 1]]
print(m[1:2, :]) # [[2 3]]

# vertical
print(m[:, 0:1]) # [[0] [2]]
print(m[:, 1:2]) # [[1] [3]]

Note that here, because of size 2

m[0:1, :] == m[:1, :]
m[1:2, :] == m[1:, :]