accumulate fun

ex2.2.scm

@@ -0,0 +1,40 @@
+(define (make-segment start-point end-point)
+  (cons start-point end-point))
+
+(define (start-segment s)
+  (car s))
+
+(define (end-segment s)
+  (cdr s))
+
+(define (make-point x y)
+  (cons x y))
+
+(define (x-point p)
+  (car p))
+
+(define (y-point p)
+  (cdr p))
+
+(define (print-point p)
+  (newline)
+  (display "(")
+  (display (x-point p))
+  (display ",")
+  (display (y-point p))
+  (display ")"))
+
+(define (midpoint-segment s)
+  (make-point
+   (average (x-point (start-segment s))
+            (x-point (end-segment s)))
+   (average (y-point (start-segment s))
+            (y-point (end-segment s)))))
+
+(define (average x y)
+  (/ (+ x y) 2))
+
+(print-point
+ (midpoint-segment
+  (make-segment (make-point 1 1)
+                (make-point 10 20))))

ex2.33.scm

@@ -0,0 +1,18 @@
+(define (accumulate op initial sequence)
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (accumulate op initial (cdr sequence)))))
+
+(define nil '())
+
+(define (accu-map p sequence)
+  (accumulate (lambda (x y) (cons (p x) y))
+              nil
+              sequence))
+
+(define (accu-append seq1 seq2)
+  (accumulate cons seq2 seq1))
+
+(define (accu-length sequence)
+  (accumulate (lambda (x y) (+ 1 y)) 0 sequence))