Python：提取2D numpy数组的核心

Question

Say I have a 2D numpy array like this:

In[1]: x
Out[1]:
array([[0, 0, 0, 0, 0],
       [1, 1, 1, 1, 1],
       [2, 2, 2, 2, 2],
       [3, 3, 3, 3, 3],
       [4, 4, 4, 4, 4],
       [5, 5, 5, 5, 5]], dtype=int64)

and I want to extract the (n-1)*(m-1) core, which would be:

array([[1, 1, 1],
       [2, 2, 2],
       [3, 3, 3],
       [4, 4, 4]], dtype=int64)

How could I do this, since the data structure is not flat ? Do you suggest flattening it first?

This is a simplified version of a much bigger array, which core has dimension (n-33)*(n-33) .

Answer 1

You can use negative stop indices to exclude the last x rows/columns and normal start indices:

>>> x[1:-1, 1:-1]
array([[1, 1, 1],
       [2, 2, 2],
       [3, 3, 3]], dtype=int64)

For your new example:

>>> t = np.array([[0, 0, 0, 0, 0],
                  [1, 1, 1, 1, 1],
                  [2, 2, 2, 2, 2],
                  [3, 3, 3, 3, 3],
                  [4, 4, 4, 4, 4],
                  [5, 5, 5, 5, 5]], dtype=np.int64)
>>> t[1:-1, 1:-1]
array([[1, 1, 1],
       [2, 2, 2],
       [3, 3, 3],
       [4, 4, 4]], dtype=int64)

You could also remove 2 leading and trailing columns:

>>> t[1:-1, 2:-2]
array([[1],
       [2],
       [3],
       [4]], dtype=int64)

or rows:

>>> t[2:-2, 1:-1]
array([[2, 2, 2],
       [3, 3, 3]], dtype=int64)