You forgot to call collatz-iter
. Also, it's not clear what do you intend to do with counter
, you just increment it, but never actually use its value - your procedure will always return 1
(assuming that the Collatz conjecture is true, which seems quite possible).
I'm guessing you intended to return the counter, so here's how to fix your procedure:
(define (collatz n)
(define (collatz-iter n counter)
(if (<= n 1)
counter ; return the counter
(if (even? n)
(collatz-iter (/ n 2) (+ counter 1))
(collatz-iter (+ (* n 3) 1) (+ counter 1)))))
(collatz-iter n 1)) ; call collatz-iter
And this is how it works for the examples in wikipedia:
(collatz 6)
=> 9
(collatz 11)
=> 15
(collatz 27)
=> 112
So basically we're counting the length of the Collatz sequence for a given number.