Pascal Costanza wrote:

I don't need a general purpose transformation, just some guidelines to follow when I see code like this.

General guideline: Look for a solution that uses LOOP. ;)

(defun diff3 (list)
(let ((avg (loop for element in list
sum element into sum
count t into length
finally (return (/ sum length)))))
(loop for element in list
collect (- element avg))))

Gauche Scheme

(define (diff3 lst :optional (seen '()) (sum 0) (cnt 0))
(if (null? lst)
(let1 avg (/ sum cnt)
(map (cut - <> avg) (reverse seen)))
(let1 x (car lst)
(diff3 (cdr lst) (cons x seen) (+ sum x) (+ cnt 1)))))

(diff3 '(1 2 3 4 5))
===>
(-2 -1 0 1 2)

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XPost: comp.lang.scheme

Pascal Costanza wrote:

I don't need a general purpose transformation, just some guidelines to follow when I see code like this.

General guideline: Look for a solution that uses LOOP. ;)

(defun diff3 (list)
(let ((avg (loop for element in list
sum element into sum
count t into length
finally (return (/ sum length)))))
(loop for element in list
collect (- element avg))))

Gauche Scheme

(define (diff3 lst)
(let ((len 0) (sum 0))
(dolist (x lst) (inc! len) (inc! sum x))
(map (cute - <> (/ sum len)) lst)))

(diff3 '(1 2 3 4 5))
===>
(-2 -1 0 1 2)

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XPost: comp.lang.scheme

On 2024-08-30, B. Pym <Nobody447095@here-nor-there.org> wrote:

Pascal Costanza wrote:

I don't need a general purpose transformation, just some guidelines to
follow when I see code like this.

General guideline: Look for a solution that uses LOOP. ;)

(defun diff3 (list)
(let ((avg (loop for element in list
sum element into sum
count t into length
finally (return (/ sum length)))))
(loop for element in list
collect (- element avg))))

Gauche Scheme

(define (diff3 lst)
(let ((len 0) (sum 0))
(dolist (x lst) (inc! len) (inc! sum x))
(map (cute - <> (/ sum len)) lst)))

The loop algorithm contains no explicitly visible side effects
whereas yours has ugly variable stepping.

Point-free one-liner:

This is the TXR Lisp interactive listener of TXR 296.
Quit with :quit or Ctrl-D on an empty line. Ctrl-X ? for cheatsheet.
TXR kind of supports IEEE 754.00000000000003 floating-point.

[[callf mapcar [chain [callf / sum len] (do op - @@1)] identity] '(1

2 3 4 5)]
(2.0 1.0 0.0 -1.0 -2.0)

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

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XPost: comp.lang.scheme

B. Pym wrote:

Pascal Costanza wrote:

I don't need a general purpose transformation, just some guidelines to follow when I see code like this.

General guideline: Look for a solution that uses LOOP. ;)

(defun diff3 (list)
(let ((avg (loop for element in list
sum element into sum
count t into length
finally (return (/ sum length)))))
(loop for element in list
collect (- element avg))))

Gauche Scheme

(define (diff3 lst)
(let ((len 0) (sum 0))
(dolist (x lst) (inc! len) (inc! sum x))
(map (cute - <> (/ sum len)) lst)))

(diff3 '(1 2 3 4 5))
===>
(-2 -1 0 1 2)

Since it's "cute" instead of "cut", (/ sum len) is done only once.

"Cute is a variation of cut that evaluates expr-or-slots before
creating the procedure."

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