;; slide no. 12

(car '(a b c))
; a

(car '((a) b (c d)))
; (a)

(cdr '(a b c))
; (b c)

(cdr '((a) b (c d)))
; (b (c d))



;; slide no. 13
(car (cdr (cdr '((a) b (c d)))))
; (c d)

(caddr '((a) b (c d)))
; (c d)

(cons 'a ())
; (a)

(cons 'd '(e))
; (d e)

(cons '(a b) '(c d))
; ((a b) c d)

(cons '(a b c) '((a) b))
; ((a b c) (a) b)


;; slide no. 20 - Typos on slide - should be 42 not just 2 in results.

(list 1 42 #t #f ())
; (1 42 #t () ())

(list '1 '42 '#t '#f '())
; (1 42 #t () ())



;; slide no. 30
;; sum 1 + 2 + ... + n
(define (sum-n n) 
  (cond ((zero? n) 0)
        (else (+ n (sum-n (- n 1))))
  )
)


;; slide no. 31
;;length of a list
(define (my_length x) 
  (cond ((null? x) 0)
        (else (+ 1 (my_length (cdr x))))
  )
)


;; slide no. 33
;; absolute values of elements in list (superficial level)
(define (abs-list x) 
  (cond ((null? x) ())
        (else (cons (abs (car x)) (abs-list (cdr x))))
  )
)


;; slide no. 34
;; appends two lists
(define (my_append x y) 
  (cond ((null? x) y)
        (else (cons (car x) (my_append (cdr x) y)))
  )
)