Solution to 2.3

diff --git a/src/sicp/ex2_3.clj b/src/sicp/ex2_3.clj
new file mode 100644 (file)
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+++ b/src/sicp/ex2_3.clj
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+(ns sicp.ex2_3
+  (:use [sicp ex2_2 utils]
+       [clojure.test]))
+
+(declare width-rect height-rect)
+;; wishful thinking. Assume that constructors and selectors are
+;; available. Now compute perimeter.
+(defn perimeter-rect [rect]
+  (* 2 (+ (width-rect rect)
+         (height-rect rect))))
+
+(defn area-rect [rect]
+  (* (width-rect rect)
+     (height-rect rect)))
+
+;; now implement the rectangle. One way to represent a rectangle
+;; is to use a pair of points representing diagonal corners.
+(defn make-rect [p1 p2]
+  (list p1 p2))
+
+(defn p1-rect [r]
+  (first r))
+
+(defn p2-rect [r]
+  (first (rest r)))
+
+(defn width-rect [r]
+  (let [p1 (p1-rect r)
+       p2 (p2-rect r)]
+    (abs (- (x-point p1) (x-point p2)))))
+
+(defn height-rect [r]
+  (let [p1 (p1-rect r)
+       p2 (p2-rect r)]
+    (abs (- (y-point p1) (y-point p2)))))
+
+(deftest test-rect-perimeter-square
+  (is (= (perimeter-rect (make-rect (make-point 0 0) (make-point 1 1))) 4)))
+
+(deftest test-rect-area-square
+  (is (= (area-rect (make-rect (make-point 0 0) (make-point 2 2))) 4)))
+
+;; part2 - another implementation
+;; instead of coordinates, we store height and width itself.
+(defn make-rect-1 [h w]
+  (list h w))
+
+(defn width-rect-1 [r]
+  (first r))
+
+(defn height-rect-1 [r]
+  (first (rest r)))
+
+;; the below routines are unmodified versions of the above
+;; procedures, but renamed to work around the compiler. Is
+;; there a simpler way?
+(defn perimeter-rect-1 [rect]
+  (* 2 (+ (width-rect-1 rect)
+         (height-rect-1 rect))))
+
+(defn area-rect-1 [rect]
+  (* (width-rect-1 rect)
+     (height-rect-1 rect)))
+
+(deftest test-rect-perimeter-square-1
+ (is (= (perimeter-rect-1 (make-rect-1 1 1)) 4)))
+
+(deftest test-rect-area-square-1
+ (is (= (area-rect-1 (make-rect-1 2 2)) 4)))