I'm currently working my way through exercise 3.17 of SICP. I know that I'm messing it up and I intend to fix that later. I will not link the exercise, as it is not relevant. This was one of my attempts:
#lang sicp
(define (count-pairs x)
(define encountered '())
(define counter 0)
(define (loop x)
(set! counter (+
(cond
((null? x) 0)
((not (pair? x)) 0)
((null? encountered) (set! encountered (list (car x))) 1)
((eq? (car x) (car encountered)) 0)
(else 1))
counter))
(if (not (pair? x)) counter (begin (loop (car x))
loop (cdr x))))
(loop x))
(count-pairs (list 'a 'b 'c))
(define second (cons 'a 'b))
(define third (cons 'a 'b))
(define first (cons second third))
(set-car! third second)
(count-pairs first)
(define 3rd (cons 'a 'b))
(define 2nd (cons 3rd 3rd))
(define 1st (cons 2nd 2nd))
(count-pairs 1st)
To my shock, this returned:
(b c)
((a . b) . b)
((a . b) a . b)
How is this possible? I know that this code isn't even close to doing as intended, but as far as I can see it should only do arithmetic and therefore return numbers. How is it possible for this code to return list structures?
Get a new IDE. Stack Overflow's syntax highlighting makes this a dead giveaway.
(begin (loop (car x))
loop (cdr x))))