Retrieving perimeter points of point in Python list comprehension-CodePudding

Given p(y,x), how do I retrieve perimeter points with offset 1 [(y-1,x), (y 1,x 1), (y,x 1),(y 1,x 1), (y 1,x), (y-1,x-1), (y,x-1), (y-1,x-1)] in list comprehension in Python?

|------------|----------|------------|
| (y-1, x-1) | (y-1,x)  | (y-1, x 1) |
|------------|----------|------------| 
| (y, x-1)   | (y,x)    | (y, x 1)   |
|------------|----------|------------|
| (y 1, x-1) | (y 1, x) | (y 1, x 1) |
|------------|----------|------------|

CodePudding user response：

Just for fun, using complex numbers:

[(round(z.imag), round(z.real))
 for a in range(8)
 for z in [complex(x, y)   1j**(a/2)]]

CodePudding user response：

The perimeter points are all of the form (x dx, y dy) for suitable values of dx and dy.

What are those suitable values? 1, 0, and -1.

from itertools import product


pp = [(x dx, y dy) for dx, dy in product([-1, 0, 1], repeat=2) if dx != 0 or dy != 0]

The filter prevent you from adding (x,y) itself to the result (i.e., when dx == 0 and dy == 0).