Note: I am using R5RS
(define (found? value L)
(cond
((null? L) #f)
((equal? (car L) value) #t)
(else (found? value (cdr L)))))
This is the function I wrote for iterating over a list like (7 "a" (b (c a) () (d d))).
If I call (found? "a" '(7 "a" (b (c a)) () (d d))), it returns #t.
But if I call (found? 'a '(7 "a" (b (c a)) () (d d))), it returns #f.
I know that if I look for the entire sublist like (b (c a)), the function returns true.
But it doesn't work for nested sublist, e.g. (c a).
How can I find the a within the sublist (c a)?
You have to add an additional cond branch for nested lists:
(define (found? value lst)
(cond
((null? lst) #f)
((equal? (car lst) value) #t)
((list? (car lst))
(or (found? value (car lst))
(found? value (cdr lst))))
(else (found? value (cdr lst)))))
Tests:
> (found? "a" '(7 "a" (b (c a)) () (d d)))
#t
> (found? 'a '(7 "a" (b (c a)) () (d d)))
#t