1 ;; Exercise 3.18.  Write a procedure that examines a list and determines whether it contains a cycle, that is, whether a program that tried to find the end of the list by taking successive cdrs would go into an infinite loop. Exercise 3.13 constructed such lists. 
2 
3 (define (cycle? l)
4   (let ((traversed '()))
5     (define (not-all-unique? l)
6       (cond ((not (pair? l)) #f)
7 	    ((memq l traversed) #t)
8 	    (else (set! traversed (cons l traversed))
9 		  (not-all-unique? (cdr l)))))
10     (not-all-unique? l)))
11 
12 (define (test-case actual expected)
13   (newline)
14   (display "Actual:   ")
15   (display actual)
16   (newline)
17   (display "Expected: ")
18   (display expected)
19   (newline))
20 
21 (define (last-pair x)
22   (if (null? (cdr x))
23       x
24       (last-pair (cdr x))))
25 
26 (define (make-cycle x)
27   (set-cdr! (last-pair x) x)
28   x)
29 
30 (define three '(a b c))
31 (define a-pair (cons '() '()))
32 (define b-pair (cons a-pair a-pair))
33 (define four (cons 'a b-pair))
34 (define seven (cons b-pair b-pair))
35 (define circular (make-cycle '(a b c)))
36 
37 (test-case (cycle? three) #f)
38 (test-case (cycle? four) #f)
39 (test-case (cycle? seven) #f)
40 (test-case (cycle? circular) #t)