sicp2.22 An iterative version of the answer that is easier to understand

2.22.scm

; My answer.

#|
keypoints:
* cache tail node.
* call set-cdr!

ref:
* tips: cons build pairs, not lists. cons build pairs, not lists.   ref: https://stackoverflow.com/a/5741668
* https://medium.com/@aleksandrasays/my-other-car-is-a-cdr-3058e6743c15
|#

(define (iter things answer tail)
    (if (null? things)
        answer
        (let ((new-tail (list (square (car things)))))
            (iter (cdr things)
                (if (null? answer)
                        new-tail
                        (begin
                            (set-cdr! tail new-tail)
                            answer
                        )
                )
                new-tail
            )
        )
    )
)

(trace iter)

(define (square-list items)
    (iter items nil nil)
)

#|
|(iter (1 2 3 4) () ())
|(iter (2 3 4) (1) (1))
|(iter (3 4) (1 4) (4))
|(iter (4) (1 4 9) (9))
|(iter () (1 4 9 16) (16))
|(1 4 9 16)
(1 4 9 16)
|#
(square-list (list 1 2 3 4))