common-lisp - 在二维列表中寻找邻居，聪明的方式

Question

让我们假设我有一个二维列表(如果你愿意，也可以是数组)，如下所示:

'( (I I I O)
   (I X I O)
   (I I I O))

现在让我们假设我想找到 X 的所有邻居。在这种情况下，我的函数将返回一个包含 8 个 I:s 的列表。我将如何以一种聪明的方式实现这个功能？我已经实现了一个如下所示的功能:

(defun get-neighbours (board x y)
  (let ((output '() ))
    (push (get-if-possible board (1+ x) y) output)
    (push (get-if-possible board (1- x) y) output)
    (push (get-if-possible board x (1+ y)) output)
    (push (get-if-possible board x (1- y)) output)
    (push (get-if-possible board (1+ x) (1+ y)) output)
    (push (get-if-possible board (1- x) (1- y)) output)
    (push (get-if-possible board (1+ x) (1- y)) output)
    (push (get-if-possible board (1- x) (1+ y)) output)
  output))

那太……丑陋了。

Answer 1

准备了这样的东西:

(defconstant +neighbour-offsets+
       '((-1 . -1) (0 . -1) (1 . -1)
         (-1 .  0)          (1 .  0)
         (-1 .  1) (0 .  1) (1 .  1)))

然后你的函数可以像这样简单

(defun get-neighbours (board x y)
  (mapcar (lambda (offs)
            (let ((dx (car offs))
                  (dy (cdr offs)))
              (get-if-possible board (+ x dx) (+ y dy))))
          +neighbour-offsets+))