From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Tue, 6 Apr 2010 08:03:33 +0000 (+0530)
Subject: added solution to exercise 1.11
X-Git-Url: https://git.rkrishnan.org/?a=commitdiff_plain;h=84c18e32eb1e688ab700b39f71d9da7abbb3c69d;p=sicp.git

added solution to exercise 1.11
---

diff --git a/chapter1/ch1_2.clj b/chapter1/ch1_2.clj
index 6780735..19a3e99 100644
--- a/chapter1/ch1_2.clj
+++ b/chapter1/ch1_2.clj
@@ -51,4 +51,224 @@
   TRACE t2806: (factorial2 6)
   TRACE t2806: => 720
   720
-  )
\ No newline at end of file
+  )
+
+(defn fact-iter [product counter max-count]
+  (if (> counter max-count)
+    product
+    (fact-iter (* product counter)
+	       (inc counter)
+	       max-count)))
+
+(defn factorial [n]
+  (fact-iter 1 1 n))
+
+(comment
+user> (dotrace [factorial fact-iter] (factorial 6))
+TRACE t2378: (factorial 6)
+TRACE t2379: |    (fact-iter 1 1 6)
+TRACE t2380: |    |    (fact-iter 1 2 6)
+TRACE t2381: |    |    |    (fact-iter 2 3 6)
+TRACE t2382: |    |    |    |    (fact-iter 6 4 6)
+TRACE t2383: |    |    |    |    |    (fact-iter 24 5 6)
+TRACE t2384: |    |    |    |    |    |    (fact-iter 120 6 6)
+TRACE t2385: |    |    |    |    |    |    |    (fact-iter 720 7 6)
+TRACE t2385: |    |    |    |    |    |    |    => 720
+TRACE t2384: |    |    |    |    |    |    => 720
+TRACE t2383: |    |    |    |    |    => 720
+TRACE t2382: |    |    |    |    => 720
+TRACE t2381: |    |    |    => 720
+TRACE t2380: |    |    => 720
+TRACE t2379: |    => 720
+TRACE t2378: => 720
+720
+)
+
+;; observation: clojure loop-recur construct is essentially the same as
+;; the iterative version.
+
+;; exercise 1.9
+(defn ++ [a b]
+  (if (= a 0)
+    b
+    (inc (++ (dec a) b))))
+
+(comment
+  This version is a recursive process, where the previous call increments
+  the sum by 1 and each call decrement the first operand by 1.
+  
+user> (dotrace [++] (++ 4 5))
+TRACE t3745: (++ 4 5)
+TRACE t3746: |    (++ 3 5)
+TRACE t3747: |    |    (++ 2 5)
+TRACE t3748: |    |    |    (++ 1 5)
+TRACE t3749: |    |    |    |    (++ 0 5)
+TRACE t3749: |    |    |    |    => 5
+TRACE t3748: |    |    |    => 6
+TRACE t3747: |    |    => 7
+TRACE t3746: |    => 8
+TRACE t3745: => 9
+9
+)
+(defn ++ [a b]
+  (if (= a 0)
+    b
+    (++ (dec a) (inc b))))
+
+(comment
+  
+user> (dotrace [++] (++ 4 5))
+TRACE t3766: (++ 4 5)
+TRACE t3767: |    (++ 3 6)
+TRACE t3768: |    |    (++ 2 7)
+TRACE t3769: |    |    |    (++ 1 8)
+TRACE t3770: |    |    |    |    (++ 0 9)
+TRACE t3770: |    |    |    |    => 9
+TRACE t3769: |    |    |    => 9
+TRACE t3768: |    |    => 9
+TRACE t3767: |    => 9
+TRACE t3766: => 9
+9
+)
+
+;; exercise 1.10
+;; ackerman functions
+(defn A [x y]
+  (cond (= y 0) 0
+	(= x 0) (* 2 y)
+	(= y 1) 2
+	:else (A (- x 1)
+		 (A x (- y 1)))))
+
+(comment
+user> (A 1 10)
+1024
+user> (A 2 4)
+65536
+user> (A 3 3)
+65536
+)
+
+(defn f [n] (A 0 n)) ; f(n) = 2n
+(defn g [n] (A 1 n)) ; g(n) = 2^n
+(comment
+  g (n) = A (1,n)
+        = A (0, A (1, n-1)) = f (A(1,n-1))
+	  = f (f (1,n-2)) 
+	    .....
+	    = f (f (f ... f (1,(n- (n-1)))))
+	      = f (f (f ... 2))
+		= 2 * (2^(n-1))
+		  = 2^n
+)
+
+(defn h [n] (A 2 n)) ; h(n) = 2^(n^2)
+
+;; section 1.2.2: Tree Recursion
+(defn fib [n]
+  (cond (= n 0) 0
+	(= n 1) 1
+	:else (+ (fib (- n 1))
+		 (fib (- n 2)))))
+
+;; iterative version
+(defn fib [n]
+  (fib-iter 1 0 n))
+
+(defn fib-iter [a b count]
+  (if (= count 0)
+    b
+    (fib-iter (+ a b) a (dec count))))
+
+;; example: counting changes
+(defn count-change [amount]
+  (cc amount 5))
+
+(defn cc [amount kinds-of-coins]
+  (cond (= amount 0) 1
+	(or (< amount 0) (= kinds-of-coins 0)) 0
+	:else (+ (cc amount (- kinds-of-coins 1))
+		 (cc (- amount
+			(first-denomination kinds-of-coins))
+		     kinds-of-coins))))
+
+(defn first-denomination [kinds-of-coins]
+  (cond (= kinds-of-coins 1) 1
+	(= kinds-of-coins 2) 5
+	(= kinds-of-coins 3) 10
+	(= kinds-of-coins 4) 25
+	(= kinds-of-coins 5) 50))
+(defn first-denomination [kinds-of-coins]
+  ({1 1 2 5 3 10 4 25 5 50} kinds-of-coins))
+
+;; exercise 1.11: A function f is defined by the rule that f(n) = n if n < 3
+;;                and f(n) = f(n - 1) + 2f(n  - 2) + 3f(n - 3) if n> 3.
+;;                Write a procedure that computes f by means of a recursive
+;;                process. Write a procedure that computes f by means of an
+;;                iterative process. 
+(defn f [n]
+  (if (< n 3)
+    n
+    (+ (f (- n 1))
+       (* 2 (f (- n 2)))
+       (* 3 (f (- n 3))))))
+
+(comment
+user> (map f (range 10))
+(0 1 2 4 11 25 59 142 335 796)  
+)
+
+;; ex 1.11: iterative version
+(defn f [n]
+  (if (< n 3)
+    n
+    (f-iter n 2 1 0)))
+
+(defn f-iter [count prev0 prev1 prev2]
+  (if (= count 3)
+    (+ prev0
+       (* 2 prev1)
+       (* 3 prev2))
+    (f-iter (dec count)
+	    (+ prev0
+	       (* 2 prev1)
+	       (* 3 prev2))
+	    prev0
+	    prev1)))
+
+;; ex 1.11: iterative version with let
+(defn f-iter [count prev0 prev1 prev2]
+  (let [res (+ prev0 (* 2 prev1) (* 3 prev2))]
+    (if (= count 3)
+      res
+      (f-iter (dec count)
+	      res
+	      prev0
+	      prev1))))
+
+;; 1.2.4: exponentiation
+;; computing b^n
+(defn expt [b n]
+  (if (= n 0)
+    1
+    (* b (expt b (dec n)))))
+
+;; iterative
+(defn expt [b n]
+  (expt-iter b n 1))
+
+(defn expt-iter [base counter product]
+  (if (= counter 0)
+    product
+    (expt-iter b
+	       (dec counter)
+	       (* product base))))
+
+;; fast version
+(defn fast-expt [b n]
+  (cond (= n 0) 1
+	(even? n) (square (fast-expt b (/ n 2)))
+	:else (* b (fast-expt b (dec n)))))
+
+(defn even? [x]
+  (= (rem x 2) 0))
\ No newline at end of file