zestime

>>> import re
>>> msg1 = "aaabbbccc"
>>> msg2 = "dddbbbeee"
>>> re.findall("bbb(?=ccc)", msg1) # lookahead
['bbb']
>>> re.findall("bbb(?=ccc)", msg2)
[]
>>> re.findall("bbb(?!ccc)", msg1) # negative lookahead
[]
>>> re.findall("bbb(?!ccc)", msg2)

zestime

(define (make-account balance key)
  (let ((try 0))
    (define (withdraw amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))
    (define (deposit amount)
      (set! balance (+ balance amount))
      balance)

zestime

(define (make-account balance key)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch k m)

zestime

(define (make-accumulator init)
  (lambda (value)
    (begin (set! init (+ init value))
           init)))

zestime

; No change, as same as before
(define (element-of-set? x set)
  (cond ((null? set) false)
        ((equal? x (car set)) true)
        (else (element-of-set? x (cdr set)))))

; Just use cons, don't need to use element-of-set anymore
(define (adjoin-set x set)
  (cons x set))

zestime

(define (element-of-set? x set)
  (cond ((null? set) false)
        ((equal? x (car set)) true)
        (else (element-of-set? x (cdr set)))))


(define (adjoin-set x set)
  (if (element-of-set? x set)
      set
      (cons x set)))

zestime

(define (myequal? col1 col2)
  (if (and (pair? col1) (pair? col2))
      (and (eq? (car col1) (car col2)) (myequal? (cdr col1) (cdr col2)))
      (eq? col1 col2)))
 
(myequal? '(this is a list) '(this is a list))  ; #t
(myequal? '(this is a list) '(this (is a) list))  ; #f

zestime

(list 'a 'b 'c) ; '(a b c)
(list (list 'george)) ; '((george))
(cdr '((x1 x2) (y1 y2)))  ; '((y1 y2))
(cadr '((x1 x2) (y1 y2))) ; '(y1 y2)
(pair? (car '(a short list))) ; #f
(memq 'red '((red shoes) (blue socks))) ; #f
(memq 'red '(red shoes blue socks)) ; '(red shoes blue socks)