I'm a "Nil" or () in Lisp World.

I wanted to get a list of all nodes in edge list and I wrote a code to solve this problem. But I met some unexpected problem.

(Codes from 'Land of Lisp' - chapter 8)

```
;; Creating edge list
(defun random-node ()
(1+ (random *node-num*)))
(defun edge-pair (a b)
(unless (eql a b)
(list (cons a b) (cons b a))))
(defun make-edge-list ()
(apply #'append (loop repeat *edge-num*
collect (edge-pair (random-node) (random-node)))))
(defparameter el (make-edge-list))
```

I wrote a code to extract all node as a list like below.

```
;; el : list of dotted list
;; I want to extract all the first element from every dotted lists in el.
;; el : ((25 . 6) (6 . 25) (2 . 13) (13 . 2) (25 . 16) (16 . 25) ....)
;; What I want to get: (25 6 2 13 25 16 ... )
(defun extract-nodes (el)
(let ((nodes nil))
(labels ((addNode (edgeList)
(push (caar edgeList) nodes)
(addNode (cdr edgeList))))
(addNode el))
nodes))
```

I thought that my code was not so bad, but result showed me a embarrassing error message.

```
"Stack overflow (deep)"
```

Stack overflow? I think that it is caused by the recursive function in my code. How can I fix this properly?

Thanks.

Answer:

Your recursive `addNode`

(better called `add-node`

if you are a lisper) needs a stop condition. E.g.,

```
(add-node (edge-list)
(push (car (pop edge-list)) nodes)
(when edge-list
(add-node (cdr edge-list))))
```

Note that there is no reason to use recursion here, a simple `mapcar`

would do just fine:

```
(defun extract-nodes (el)
(mapcar #'car el))
```

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