Question

I am reading a programming languages book and it is asking me to to explain what the following Scheme function does (not sure, can someone help explain):

(define (x lis)
  (cond ((null? lis) 0) 
        ((not (list? (car lis))) 
         (cond 
          ((eq? (car lis) #f) (x (cdr lis))) 
          (else (+ 1 (x (cdr lis)))))) 
        (else (+ (x (car lis)) (x (cdr lis))))))

Answer 1

It counts the number of leaf nodes a nested list structure, ignoring #f. It uses a recursive procedure:

• If the input is an empty list, this is the base case that returns 0.
• If the input is a pair whose car is not a list:
  • If the car is #f, we recurse on the cdr and return that.
  • Otherwise we count the car as 1, and add that to the result of recursing on the cdr.
• Finally, we recurse on both the car and cdr, and add them together.