How to get all array edges?

Question

I have a n x n array, and want to receive its outline values. For example,

[4,5,6,7]

[2,2,6,3]

Answer 1

In [1]: arr=np.arange(16).reshape(4,4)
In [2]: arr
Out[2]: 
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])

A relatively straight forward way of doing this - in clockwise order is:

In [5]: alist=[arr[0,:-1], arr[:-1,-1], arr[-1,::-1], arr[-2:0:-1,0]]
In [6]: alist
Out[6]: [array([0, 1, 2]), array([ 3,  7, 11]), array([15, 14, 13, 12]), array([8, 4])]
In [7]: np.concatenate(alist)
Out[7]: array([ 0,  1,  2,  3,  7, 11, 15, 14, 13, 12,  8,  4])

In a sense it's a loop, in that I have to build 4 slices. But if 4 is small compared to n, that's a small price. It has to concatenate at some level.

If order doesn't matter we could simplify the slices some (e.g. forgetting the reverse order, etc).

alist=[arr[0,:], arr[1:,-1], arr[-1,:-1], arr[1:-1,0]]

If I didn't care about order, or double counting the corners I could use:

np.array([arr[[0,n],:], arr[:,[0,n]].T]).ravel()

eliminating the duplicate corners

In [18]: np.concatenate((arr[[0,n],:].ravel(), arr[1:-1,[0,n]].ravel()))
Out[18]: array([ 0,  1,  2,  3, 12, 13, 14, 15,  4,  7,  8, 11])