# [CM] one-over-f-noise

**Carl Edwards
**
carl.boingie@rcn.com

*Mon, 11 Nov 2002 00:08:58 -0500*

If anyone has time to answer a simple Lisp question:
I've been looking at an old example that I found online, the
one-over-f-noise.lsp function (or pair of functions). There is a part I
don't understand in the let* form (if that's the right word) of the
second function. Here's the listing:
(defun one-over-f-noise (power-of-2)
(let* (rtn
(length (expt 2 power-of-2))
(half-range (/ 1.0 power-of-2))
(r (make-array (list power-of-2)))) ; bug: should free array
(loop for n from 0 to (- length 1) do
(push
(one-over-f-aux n power-of-2 r half-range)
rtn))
rtn))
I'll append the whole program below (necessary if you want to try it at
home).
My question is regarding the variable (or function) rtn. Is "rtn" short for
something that I can't find in cltl2? Just point me in the right direction,
I just need to know what to look up.
Another sub-question is: should I worry about the documented bug? The
program seems to work just fine.
Thanks in advance,
Carl Edwards
---------------snip-----------------------------
(in-package :cm)
(defun one-over-f-aux (n power-of-2 r half-range)
(let ((sum 0))
(loop for i below power-of-2
do
(let ((pow (expt 2 i)))
(when (not (= (/ n pow) (/ (- n 1) pow)))
(setf (aref r i) (between (- half-range) half-range)))
(incf sum (aref r i))))
sum))
;;; One over f noise generator. Returns a list of size 2^power-of-2 of
;;; numbers from -1 to 1
;;; Based on Gardner (1978) and Dick Moore (1988?)
(defun one-over-f-noise (power-of-2)
(let* (rtn
(length (expt 2 power-of-2))
(half-range (/ 1.0 power-of-2))
(r (make-array (list power-of-2)))) ; bug: should free array
(loop for n from 0 to (- length 1) do
(push
(one-over-f-aux n power-of-2 r half-range)
rtn))
rtn))
----------------------snip-------------------------------------