Added Ex 3.3

scheme/week10/ex3_3.scm

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+;; SICP 3.3
+
+;; Exercise 3.3.  Modify the make-account procedure so that it creates
+;; password-protected accounts. That is, make-account should take a
+;; symbol as an additional argument, as in
+
+;; (define acc (make-account 100 'secret-password))
+
+;; The resulting account object should process a request only if it is
+;; accompanied by the password with which the account was created, and
+;; should otherwise return a complaint: 
+
+;; ((acc 'secret-password 'withdraw) 40)
+;; 60
+
+;; ((acc 'some-other-password 'deposit) 50)
+;; "Incorrect password"
+
+;; -------------------------------------------------------------------
+
+(define (make-account actual-password balance)
+  (define (withdraw amount)
+    (cond ((> amount balance) "Insufficient funds")
+          (else (set! balance (- balance amount))
+                balance)))
+  (define (deposit amount)
+    (set! balance (+ balance amount))
+    balance)
+  (define (bad-password ignored)
+    "Incorrect password")
+  (lambda (password operation)
+    (cond ((not (equal? password actual-password)) bad-password)
+          ((equal? operation 'withdraw) withdraw)
+          ((equal? operation 'deposit) deposit))))

scheme/week10/ex3_3_test.scm

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+;; SICP Tests 3.3
+
+(test-case "Ex 3.3 -- make-account creates an account"
+           (let ((a (make-account 'pw 10)))
+             (assert-not-equal () a)))
+
+(test-case "Ex 3.3 -- withdrawing money from an account"
+           (let ((acc (make-account 'pw 10)))
+             (assert-equal 8 ((acc 'pw 'withdraw) 2))))
+
+(test-case "Ex 3.3 -- withdrawing all money from an account"
+           (let ((acc (make-account 'pw 10)))
+             (assert-equal 0 ((acc 'pw 'withdraw) 10))))
+
+(test-case "Ex 3.3 -- overdrawing money from an account"
+           (let ((acc (make-account 'pw 10)))
+             (assert-equal "Insufficient funds" ((acc 'pw 'withdraw) 12))))
+
+(test-case "Ex 3.3 -- depositing money to an account"
+           (let ((acc (make-account 'pw 10)))
+             (assert-equal 17 ((acc 'pw 'deposit) 7))))
+
+(test-case "Ex 3.3 -- bad password"
+           (let ((acc (make-account 'pw 10)))
+             (assert-equal "Incorrect password"
+                           ((acc 'bad-pw 'deposit) 7))
+             (assert-equal "Incorrect password"
+                           ((acc 'bad-pw 'withdraw) 7))))