Subject: Re: how to cast integer into string ?
From: (Rob Warnock)
Date: Sun, 07 Dec 2008 23:07:49 -0600
Newsgroups: comp.lang.lisp
Message-ID: <>
Tamas K Papp  <> 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)


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			<>
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