Line segments in plane

ex2.2-segments.rkt

@@ -0,0 +1,64 @@
+#lang racket
+; Exercise 2.2.  Consider the problem of representing line segments in a plane.
+; Each segment is represented as a pair of points: a starting point and an ending point. 
+
+(require "../common.rkt")
+
+(define (make-point x y )
+  (cons x y))
+
+(define (get-x p)
+  (car p))
+
+(define (get-y p)
+  (cdr p))
+
+(define (make-segment start-p end-p)
+  (cons start-p end-p))
+
+(define (get-start segment)
+  (car segment))
+
+(define (get-end segment)
+  (cdr segment))
+
+(define (get-midpoint segment)
+  (let ((start-x (get-x (get-start segment)))
+       (end-x   (get-x (get-end segment)))
+       (start-y (get-y (get-start segment)))
+       (end-y (get-y (get-end segment))))
+    (make-point  (/ (+ start-x end-x) 2.0)
+                 (/ (+ start-y end-y) 2.0)))) 
+
+(test "make-point" 
+     (λ ()
+       (assert "x is car" (car (make-point -5 7)) -5)
+       (assert "y is cdr" (cdr (make-point -5 7)) 7)
+       (assert "is pair" (pair? (make-point -5 7)) #t)))
+
+(test "get-x"
+      (λ ()
+        (assert-simple (get-x (make-point -5 7)) -5)))
+
+(test "get-y"
+      (λ ()
+        (assert-simple (get-y (make-point -5 7)) 7)))
+
+(define (with-data)
+  (define p1 (make-point 3.0 4.0))
+  (define p2 (make-point 6.0 7.0))
+  (test "make-segment" 
+      (λ ()
+          (assert "start is car" (car (make-segment p1 p2)) p1)
+          (assert "end is cdr" (cdr (make-segment p1 p2)) p2)))
+  (test "get endpoints"
+      (λ ()
+          (assert "start" (get-start (make-segment p1 p2)) p1)
+          (assert "end" (get-end (make-segment p1 p2)) p2)))
+  (test "mid-point" 
+      (λ ()
+        (assert "x of midpoint" (get-x (get-midpoint (make-segment p1 p2))) 4.5)
+        (assert "y of midpoint" (get-y (get-midpoint (make-segment p1 p2))) 5.5)
+        (assert "y of midpoint" (get-end (make-segment p1 p2)) p2))))
+
+(with-data)