CSC151, Class 18: List Recursion, Continued

Overview:
* tally-skips
* Lab
* Reflection

Notes:
* Read Numeric Recursion
* How to make the Delete key work the way you want.
* Lab writeup 2 is optional
* List of topics updated.
* Grading writeup 1.  Done soon.
* Our Tuesday visitors return.
* No, you won't finish the lab.

Tally-Skips

(tally-skips (list 'hop 'skip 'jump 'and 'skip 'again))
=> (tally-skips (list 'skip 'jump 'and 'skip 'again))
=> (+ 1 (tally-skips (list 'jump 'and 'skip 'again)))
=> (+ 1 (tally-skips (list 'and 'skip 'again)))
=> (+ 1 (tally-skips (list 'skip 'again)))
=> (+ 1 (+ 1 (tally-skips (list 'again))))
=> (+ 1 (+ 1 (tally-skips (list))))
=> (+ 1 (+ 1 0))
=> (+ 1 1)
=> 2

; Purpose:
;   count the number of times the symbol 'skip
;   appears in lst.
(define tally-skips
  (lambda (lst)
    (cond
      ; If the list is empty [Cite: KP]
      ((null? lst) 0)
      ; If the car of the list is equal to
      ; skip [Cite: Many voices]
      ((eq? (car lst) 'skip)
       ; Count that one
       ; Count how many skips remain
       ; Add 'em together.
       (+ 1 (tally-skips (cdr lst))))
      ; If the first one isn't skip
      (else (tally-skips (cdr lst))))))

; Another version: Using a helper procedure to
; keep a running count of the number of skips
; seen so far
(define skip-tallier
  (lambda (lst skips)
    (display (list 'skip-tallier lst skips))
    (newline)
    (cond
      ((null? lst) skips)
      ((eq? (car lst) 'skip)
       ; Continue, after ...
       (skip-tallier
        ; Drop the first element of the list
        (cdr lst)
        ; Add 1 to skips
        (+ 1 skips)))
      (else
       (skip-tallier (cdr lst) skips)))))

(define dan-counts-skips
  (lambda (lst)
    (skip-tallier lst 0)))

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How do you say "Is there only one element in this list?

(if (null? (cdr lst))