;;;
;;; Part I.
;;;

; 1.)

(define x 10)
(define y 44)
(define foo '(1 2 3))
(define bar '(a b c))

x => 10
(cons x foo) => (10 1 2 3)
(cons y bar) => (44 a b c)
(list x y) => (10 44)
(list x foo) => (10 (1 2 3))
'(x y) => (x y)
(car bar) => a
(cdr bar) => (b c)
(+ x y) => 54

; 2.)

(if #f 5 10) => 10
(if (or #t #f) 5 10) => 5
(if #t
    (if #f 2 3)
    10
) 
=> 3


(define x 3)
(cond ((= x 1) 44)
      ((= x 2) (+ 5 50))
      ((= x 3) (* 6 11))
      (else    (- 80 3)))

=> 66

(let ((x 5)
      (y 10))
  (+ x y))


=> 15

; 3.)

(define (char-hex-letter? c)
            (or (char<=? #\a c #\f)
		(char<=? #\A c #\F)
))

(define (char-hex? c)
  (if (char? c)
      (or (char-hex-letter? c)
	  (char<=? #\0 c #\9))
      (error "passed char-hex? a non-character")))

;;;
;;; Part II.
;;;

; 1.)

; helper function
(define (char-hex-letter->number c)
  (if (char<=? #\a c #\f)
      (+ 10 (- (char->integer c) (char->integer #\a)))
      (+ 10 (- (char->integer c) (char->integer #\A)))))

(define (char-hex->number c)
  (if (char-hex-letter? c)
      (char-hex-letter->number c)
      (if (char-hex? c)
	  ; we know it's a number now
	  (- (char->integer c) (char->integer #\0))
	  (error "passed char-hex->number something not in [0-9a-fA-F]"))))

; 2.)

; helper function
(define (string-hex->number-iter s counter size total)
  (if (>= counter size)
      total
      (string-hex->number-iter s
	    (+ counter 1)
	    size
	    (+ (*
		16
		total)
	       (char-hex->number (string-ref s counter))))))

(define (string-hex->number s)
  (if (string? s)
      (string-hex->number-iter s 0 (string-length s) 0)
      (error "passed string-hex->number a non-string")))

; 3.)

(define (positive-only x)
  (cond 
       ((null? x) '())
       ((> (car x) 0) (cons (car x) (positive-only (cdr x))))
       (else (positive-only (cdr x)))
  )
)