# 3.1 Assignment and Local State

## Exercise 3.1

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

## Exercise 3.2

``````(define (make-monitored f)
(let ((counter 0))
(lambda (arg)
(if (eq? arg 'how-many-calls?)
counter
(begin (set! counter (inc counter))
(f arg))))))
``````

## Exercise 3.3

``````(define (make-account balance password)
(lambda (amount)
(cond ((eq? method 'deposit)
(begin (set! balance (+ balance amount))
balance))
((eq? method 'withdraw)
(if (< balance amount)
"Insufficient funds"
(begin (set! balance (- balance amount))
balance)))))
``````

## Exercise 3.4

``````(define (make-account balance password)
(let ((incorrect-times 0))
(lambda (amount)
(cond ((eq? method 'deposit)
(begin (set! balance (+ balance amount))
balance))
((eq? method 'withdraw)
(if (< balance amount)
"Insufficient funds"
(begin (set! balance (- balance amount))
balance)))))
(begin (set! incorrect-times (inc incorrect-times))
(if (>= incorrect-times 7) (call-the-cops))
``````

## Exercise 3.5

``````(define (random-in-range low high)
(+ (random (- high low)) low))

(define (estimate-integral pred x1 x2 y1 y2 trials)
(define (experiment)
(pred (random-in-range x1 x2)
(random-in-range y1 y2)))
(let ((ratio (monte-carlo trials experiment)))
(* ratio (- x2 x1) (- y2 y1))))

(define (in-unit-circle? x y)
(< (+ (* x x) (* y y)) 1.0))

(define (estimate-pi)
(estimate-integral in-unit-circle?
-1.0 1.0 -1.0 1.0
1000000)
``````

## Exercise 3.6

``````(define (rand action)
(let ((x rand-init))
(define (rand-generate)
(set! x (rand-update x))
x)
(define (rand-reset new-random-init)
(set! x new-rand-init))
(cond ((eq? action 'generate)
(rand-generate))
((eq? action 'reset)
rand-reset))))))
``````

## Exercise 3.7

Making no modifications to my solution to Exercise 3.3, I wrote `make-joint` as a “wrap” around the original account, and if `joint-password` is correct, it will provide the original password.

``````(define (make-joint account password joint-password)
``````

## Exercise 3.8

``````(define (f-obj)
(let ((last-x 0))
(lambda (x)
(let ((result last-x))
(set! last-x x)
result))))

(define f (f-obj))
``````
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