Question

I cant figure out this scheme code please help. Compute-frequencies takes in two separate lists looking-for-list and pool-list. It is supposed to return a list that shows how many times everything in looking-for-list is in pool-list.
I know I am close it is just some little error most likely having to do with the recursive call after making it through pool-list.

(define (compute-frequencies looking-for-list pool-list) 
  (define (helper looking-for-list pool-list current-frequency frequency-list)  ; keeps track of finished list and iterates through both lists
    (if (null? looking-for-list) (reverse frequency-list)                       ; finding the number of times data in looking-for-list are in pool-list
    (if (null? pool-list)
        (helper (cdr looking-for-list) pool-list 0 (cons current-frequency frequency-list))
        (if (equal? (car looking-for-list) (car pool-list))
        (helper looking-for-list (cdr pool-list) (+ 1 current-frequency) frequency-list)
        (helper looking-for-list (cdr pool-list) current-frequency frequency-list)))))

 (helper looking-for-list pool-list 0 '() ))

Answer 1

Alright reading your code it seems like your algorithm itself is perfectly fine. The problem is much simpler.

Look at your code, once you’ve checked all the elements of pool-list for the first element in your looking-for-list you want to restore pool-list to its original state. To do this, you call helper with pool-list as pool-list. You almost certainly mean the one defined in the compute-frequency arguments but Scheme takes the one in helper‘s arguments since it shadow’s the one in compute-frequency. This means that after the first iteration, pool-list is just '() so the frequency of everything else is 0.

To solve this, do some renaming. In the future, try to remember that Scheme variables can and will shadow ones in a larger scope. And also, try using cond instead of nested if statements. Much more readable in Scheme (it’s how I found this problem)