CODEWITHSUNDEEP
recursionlisp
anonuser0428
anonuser0428

Reputation: 12363

Lisp Programming Error (Stack Overflow (deep))

I am to write a lisp program to produce the actual value of a hexadecimal number. I have written a function but seem to be getting a stackoverflow (deep) error. I was wondering if anyone could point out my mistake or guide me in the right direction.

I would appreciate it if no code was posted for this question as this is part of a homework assignment. Hence I would only like an explanation or direction where I might be going wrong.

I feel my problem is that my recursion is not terminating but I don't know how to fix it.

Here's my code:

(defun calc (hex)
  (if hex
    (if (> (length hex) 1)
     ( + (first (reverse hex)) (* 16 (calc (reverse hex)))) hex)))

Thanks in advance.

Upvotes: 1

Views: 995

Answers (1)

Alexander Bird
Alexander Bird

Reputation: 40688

The "base case" (the case/state where recursion actually stops) is that hex has length of one or less. Tell me, each time you call calc again, is the input to calc ever getting smaller? if not, then it's mathematically impossible for the input to ever reach the base case.

Let's say hex starts with length 9. When you call calc again, you have reversed hex. So now hex is reversed, but it still has a length of 9. I suspect that is why the recursion never stops.

Upvotes: 3

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