Add exercises 3.1 - 3.3

3_1.rkt

@@ -0,0 +1,6 @@
+#lang sicp
+
+(define (make-accumulator sum)
+  (lambda (inc)
+    (set! sum (+ sum inc))
+    sum))

3_2.rkt

@@ -0,0 +1,12 @@
+#lang sicp
+
+(define (make-monitored f)
+  (let ((call-count 0))
+    (define (mf arg)
+      (cond ((eq? arg 'how-many-calls?) call-count)
+            ((eq? arg 'reset-count) (set! call-count 0))
+            (else
+             (set! call-count (+ call-count 1))
+             (f arg))))
+    mf))
+    

3_3.rkt

@@ -0,0 +1,20 @@
+#lang sicp
+
+(define (make-account balance account-password)
+  (define (withdraw amount)
+    (if (>= balance amount)
+        (begin (set! balance (- balance amount))
+               balance)
+        "Insufficient funds"))
+  (define (deposit amount)
+    (begin
+      (set! balance (+ balance amount))
+      balance))
+  (define (dispatch password m)
+    (if (eq? password account-password)
+        (cond ((eq? m 'withdraw) withdraw)
+              ((eq? m 'deposit) deposit)
+              (else (error "Unknown request -- MAKE-ACCOUNT"
+                           m)))
+        (error "Incorrect password")))
+  dispatch)