;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-advanced-reader.ss" "lang")((modname change_memory) (read-case-sensitive #t) (teachpacks ((lib "testing.ss" "teachpack" "htdp") (lib "draw.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #t #t none #f ((lib "testing.ss" "teachpack" "htdp") (lib "draw.ss" "teachpack" "htdp")))))

;; redefining a variable
(define x 5)
(set! x 6) ;; it's an operation, the result of it is a special
           ;; value "void"
x

;; a sequence of operations
;; note: only the last one can be non-void

(define y 2)
(begin (set! y (+ x 2))
       (set! x (+ y 2))
       (+ x y))
"x = "  
x
"y = " 
y

;; note that keeping track of variables is now much harder
;; for this reason use of memory updates should be limited

;; Address book example: a "global" address-book 

(define address-book empty)

;; lookup : symbol (listof (list symbol number))  ->  number or false
;; to lookup the phone number for name in ab
(define (lookup name ab)
  (cond
    [(empty? ab) false]
    [else (cond
	    [(symbol=? (first (first ab)) name)
	     (second (first ab))]
	    [else (lookup name (rest ab))])]))

;; add-to-address-book : symbol number  ->  void
(define (add-to-address-book name phone)
  (set! address-book (cons (list name phone) address-book)))

(add-to-address-book 'Sally 1234567)

address-book

(add-to-address-book 'Peter 9876543)

address-book

;; changing structures 
(define-struct phone-entry (name phone))

(define address-book2 (list (make-phone-entry 'Sally 1234567) (make-phone-entry 'Peter 9876543)))

address-book2


;; symbol number address-book -> void
;; Goven a name and a number, updates the phone number
;; for that name to the number in the address book
(define (update-phone name phone ab)
  (cond
    [(empty? ab) (error 'update-phone! "no such phone")]
    [(symbol=? name (phone-entry-name (first ab))) 
               (set-phone-entry-phone! (first ab) phone)]
    [else (update-phone name phone (rest ab))]
  )
)

(update-phone 'Sally 2345678 address-book2)

address-book2

(update-phone 'Peter 8765432 address-book2)

address-book2 

;; counting even numbers with a global counter
(define evens 0)

;; lon -> void
;; counts the number of even elements by updating a global counter
(define (count-evens lon)
  (cond 
    [(empty? lon) false]
    [(even? (first lon)) (begin (set! evens (+ evens 1)) (count-evens (rest lon)))]
    [else (count-evens (rest lon))]
    )
)

(count-evens (list 1 2 3 4 5 6 7 8))

"Evens:"
evens