Tamas K Papp  <tkpapp@gmail.com> wrote:
+---------------
| Others have already provided solutions.  I wanted to add mine because it 
| is important to realize that you do not need the intermediate list of 
| digits, and you can proceed from either end (+ is commutative).  So
| 
| (defun sum-digits (i)
|   (declare ((integer 0) i)) ;; we want a nonnegative integer
|   (let ((sum 0))
|     (tagbody
|      top
|        (multiple-value-bind (quotient remainder) (truncate i 10)
| 	 (incf sum remainder)
| 	 (when (plusp quotient)
| 	   (setf i quotient)
| 	   (go top))))
|     sum))
| 
| (sum-digits (expt 2 1000))
| 
| Some consider tagbody ugly - it is their loss :-)
+---------------

I cheerfully use TAGBODY when it's appropriate, but I don't consider
the above to be such a case, especially given that there's a considerably
simpler way *without* it, viz.:

    (defun sum-digits (i)
      (assert (and (integerp i) (>= i 0)))
      (loop while (plusp i)
	    summing (multiple-value-bind (quotient remainder)
			(truncate i 10)
		      (setf i quotient)
		      remainder)))


-Rob

p.s. Note that I changed your DECLARE to an ASSERT, since DECLARE is
a promise *by* the programmer, not a check *of* the programmer/user!
I suppose I could have written (ASSERT (TYPEP I '(INTEGER 0 *))),
but the TYPEP is likely to be slower.

-----
Rob Warnock			<rpw3@rpw3.org>
627 26th Avenue			<URL:http://rpw3.org/>
San Mateo, CA 94403		(650)572-2607