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

Assuming your list is in the following format:

l = [
     [4, 5, 6, 7],
     [2, 2, 6, 3],
     [4, 4, 9, 4],
     [8, 1, 6, 1]
    ]

You can achieve what you want with this quick one-liner, using list comprehensions:

out = list(l[0]) +  # [4, 5, 6, 7]
      list([i[-1] for i in l[1:-1]]) +  # [3, 4]
      list(reversed(l[-1])) +  # [1, 6, 1, 8]
      list(reversed([i[0] for i in l[1:-1]])) # [4, 2]

print(out)  # gives [4, 5, 6, 7, 3, 4, 1, 6, 1, 8, 4, 2]

This works whether you have a plain python list or a numpy array.

Regarding efficiency, using %timeit on a 20000x20000 matrix, this method took 16.4ms.

l = np.random.random(20000, 20000)
%timeit list(l[0]) + list(...) + list(...) + list(...)
100 loops, best of 3: 16.4 ms per loop

I'm sure there are more efficient methods to accomplish this task, but I think that's pretty good for a one-liner solution.