;; The original length
(define (len l)
  (if (null? l)
      0
      (+ 1 (len (cdr l)))))

;; Length using map
(define (len1 l)
  (apply + (map (lambda (x) 1) l)))

;; Atomcount from last time...
(define (atom? x) (not (pair? x)))

(define (atomcount x)
  (cond ((null? x) 0)
        ((atom? x) 1)
        (else (+ (atomcount (car x))
                 (atomcount (cdr x))))))

;; Atomcount using map and apply
(define (atomcount2 x)
  (cond ((atom? x) 1)
        (else (apply + (map atomcount2 x)))))

> (atomcount2 '(3 (4 5)))
3



(define (foldr f l base)
  (if (null? l)
      base
      (f (car l) (foldr f (cdr l) base))))

;; Length using fold
(define (len2 l)
  (foldr (lambda (x y) (+ 1 y)) l 0))

;; Atomcount using fold
(define (atomcount4 x)
  (if (atom? x)
      1
      (foldr (lambda (x y) (+ (atomcount4 x) y))
             x 0)))

;; Another atomcount using fold -- this one
;; isn't exactly the same as atomcount4 -- how it is
;; different?
(define (atomcount5 x)
  (foldr (lambda (x y) 
           (+ 
            (if (atom? x) 1 (atomcount5 x))
            y))
         x 0))