CSC151 2009F, Class 28: Recursion with Helper Procedures

Admin:
* Yes, we will have a quiz on Friday.  
  * It will be on recursion 
  * It will probably include helper recursion
* Assignment 6 due now!
* No, you don't get an assignment 7 to do over fall break.
* Reading for Friday: List Recursion, Revisited.

Overview:
* Delayed evaluation in recursive procedures.
* A strategy: Carry along intermediate results.
* Using recursive helpers.
* A term: Tail recursion.

Whiteboard (or outline) shows one way to compute sum.  We need to
code it up

(define sum
  (lambda (lst)
    (sum-as-you-go (car lst) (cdr lst))))

(define sum-as-you-go
  (lambda (partial-sum lst)
    (if (null? lst)
        partial-sum
        (sum-as-you-go (+ partial-sum (car lst)) (cdr lst)))))

* Term: Tail-recursion
  * The form of recutrsion in sum-as-you-go
  * The recursive call is the "tail" of the procedure; the last thing done