On Feb 10, 2009, at 4:15 PM, Chouser wrote:
>
>
> (defn mod42
> "Modulus of num and div. Truncates toward negative infinity."
> [num div]
> (if-not (and (integer? num) (integer? div))
> (throw (IllegalArgumentException. "mod requires two integers"))
> (let [m (rem num div)]
> (if (or (zero? m) (> (* num div) 0))
> m
> (+ m div)))))
>
Just for giggles, here's how SBCL (Steel Bank Common Lisp) defines
the two functions:
(defun rem (number divisor)
#!+sb-doc
"Return second result of TRUNCATE."
(multiple-value-bind (tru rem) (truncate number divisor)
(declare (ignore tru))
rem))
(defun mod (number divisor)
#!+sb-doc
"Return second result of FLOOR."
(let ((rem (rem number divisor)))
(if (and (not (zerop rem))
(if (minusp divisor)
(plusp number)
(minusp number)))
(+ rem divisor)
rem)))
Notice that despite the documentation, MOD does not actually call
FLOOR. It indirectly calls TRUNCATE via REM. It's also important to
notice that Common Lisp is a Lisp-2, so that we can store the result
of the REM call in a variable with the same name in the LET form.
Aloha,
David Sletten
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