Convert decimal number to octal in Lisp

Question

I'm trying to write a function in Common Lisp to convert a base 10 number into a base 8 number, represented as a list, recursively.

Here's what I have so far:

(defun base8(n)
(cond
    ((zerop (truncate n 8)) (cons n nil))
    ((t) (cons (mod n 8) (base8 (truncate n 8))))))

This function works fine when I input numbers < 8 and > -8, but the recursive case is giving me a lot of trouble. When I try 8 as an argument (which should return (1 0)), I get an error Undefined operator T in form (T).

Thanks in advance.

Answer 1

It seems you have forgotten to (defun t ...) or perhaps it's not the function t you meant to have in the cond? Perhaps it's t the truth value?

The dual namespace nature of Common Lisp makes it possible for t to both be a function and the truth value. the difference is which context you use it and you clearly are trying to apply t as a function/macro.

Here is the code edited for the truth value instead of the t function:

(defun base8(n)
  (cond
    ((zerop (truncate n 8)) (cons n nil))
    (t (cons (mod n 8) (base8 (truncate n 8))))))

(base8 8) ; ==> (0 1)

Answer 2

Just for fun, here's a solution without recursion, using built-in functionality:

(defun base8 (n)
  (reverse (coerce (format nil "~8R" n) 'list)))