Question

Is it possible to implement the R5RS scheme function “length” using the car and cdr family of functions?
If so could someone post the implementation?

Thanks,

Answer 1

Of course, it’s pretty simple. I’m not giving a straight answer because this looks like homework and anyway it’s trivial to write. Fill-in the blanks:

(define (length lst)
  (if <???>              ; if the list is empty
      <???>              ; return 0
      (<???>             ; otherwise add 1 and
       (length <???>)))) ; advance the recursion over the rest of the list

Notice that only cdr is used. We’re not interested in the actual contents of the list, so we can ignore car.