Lecture 25: Transforming into Accumulator Style

1. MAKEUP: what is backtracking?
   graph, code, what part is backtracking?
   how about making change? finding a route?

   (define (find-route O D graph visited)
     (cond
       ((eq? O D) (list D))
       ((member O visited) #f)
       (else (local ((define route
   		    (try-all-neighbors (neighbors O graph) D graph (cons O visited))))
   	    (cond
   	      ((boolean? route) #f)
   	      (else (cons O route)))))))
   
   (define (try-all-neighbors lo-Os D graph visited)
     (cond
       ((empty? lo-Os) #f)
       (else (local ((define route (find-route (first lo-Os) D graph visited)))
   	    (cond
   	      ((boolean? route) (try-all-neighbors (rest lo-Os) D graph visited))
   	      (else route))))))
   

   ---

   
   
   
2. transform an existing function into accumulator style, exercises

   (define (! n) (cond ((zero? n) 1) (else (* n (! (sub1 n))))))	(define (!-A n a) (cond ((zero? n) a) (else (!-A (sub1 n) (* a n)))))
   (define (SIGMA alon) (cond ((empty? alon) 0) (else (+ (first alon) (SIGMA (rest alon))))))	(define (SIGMA-A alon a) (cond ((empty? alon) a) (else (SIGMA-A (rest alon) (+ (first alon) a)))))
   (define (filter-non-0 alon) (cond ((empty? alon) empty) (else (cond ((zero? (first alon)) (filter-non-0 (rest alon))) (else (cons (first alon) (filter-non-0 (rest alon))))))))	(define (filter-non-0-A alon a) (cond ((empty? alon) a) (else (cond ((zero? (first alon)) (filter-non-0-A (rest alon) a)) (else (filter-non-0-A (rest alon) (cons (first alon) a)))))))

   
   
   
3. use SIGMA-A to compute average, add an accumulator

   (define (SIGMA-A alon sum-so-far num-so-far)
     (cond
       ((empty? alon) (/ sum-so-far num-so-far))
       (else (SIGMA-A (rest alon) (+ (first alon) sum-so-far) (+ 1 num-so-far)))))