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    <p>First, you are still in <em>imperative land</em>.</p> <pre><code>(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)) </code></pre> <p>You do: variable declaration, bind it to NIL, mutation of the variable, returning the variable.</p> <p>It is simply:</p> <pre><code>(defun get-neighbours (board x y) (list (get-if-possible board (1+ x) y) (get-if-possible board (1- x) y) (get-if-possible board x (1+ y)) (get-if-possible board x (1- y)) (get-if-possible board (1+ x) (1+ y)) (get-if-possible board (1- x) (1- y)) (get-if-possible board (1+ x) (1- y)) (get-if-possible board (1- x) (1+ y)))) </code></pre> <p>You can 'shorten' the code with a local macro:</p> <pre><code>(defun get-neighbours (board x y) (macrolet ((all-possible (&amp;rest x-y-combinations) `(list ,@(loop for (a b) on x-y-combinations by #'cddr collect `(get-if-posssible board ,a ,b))))) (all-possible (1+ x) y (1- x) y x (1+ y) x (1- y) (1+ x) (1+ y) (1- x) (1- y) (1+ x) (1- y) (1- x) (1+ y)))) </code></pre> <p>Now one could abstract it a bit:</p> <pre><code>(defmacro invoke-fn-on (fn &amp;rest x-y-combinations) `(funcall (function ,fn) ,@(loop for (a b) on x-y-combinations by #'cddr collect `(get-if-posssible board ,a ,b)))) (defun get-neighbours (board x y) (invoke-fn-on list (1+ x) y (1- x) y x (1+ y) x (1- y) (1+ x) (1+ y) (1- x) (1- y) (1+ x) (1- y) (1- x) (1+ y))) </code></pre> <p>About the LOOP:</p> <pre><code>&gt; (loop for (a b) on '(1 2 3 4 5 6) by #'cddr collect (list a b)) ((1 2) (3 4) (5 6)) </code></pre> <p>ON moves the pattern (a b) over the list (1 2 3 4 5 6). It would give (1 2), (2 3), (3 4), (4 5) and (5 6). With each iteration the list is moved one forward by using CDR to get the rest list. BY now says that we move by two items, by CDDR and not one item as with CDR. So we get three iterations and the pairs (1 2), (3 4) and (5 6).</p> <p>An alternative would be to slightly simplify the LOOP by introducing a different list structure for the coordinate pairs:</p> <pre><code>(defmacro invoke-fn-on (fn x-y-combinations) `(funcall (function ,fn) ,@(loop for (a b) in x-y-combinations collect `(get-if-posssible board ,a ,b)))) (defun get-neighbours (board x y) (invoke-fn-on list '(((1+ x) y ) ((1- x) y ) (x (1+ y)) (x (1- y)) ((1+ x) (1+ y)) ((1- x) (1- y)) ((1+ x) (1- y)) ((1- x) (1+ y))))) </code></pre>
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    1. POFind neighbours in 2d-list, smart way
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    1. COHm, okay, the meaning of the keywords "by" and "on" in the loop macro is still unknown to me. I still prefer angus's answer as the most readable one but I still very much appreciate your help!
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    2. COI didn't know you can do such complicated and useful things with loop. Have an up-vote!
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