code from section 2.4

diff --git a/src/sicp/ch2_4.rkt b/src/sicp/ch2_4.rkt
new file mode 100644 (file)
index 0000000..ed348fd
--- /dev/null
+++ b/src/sicp/ch2_4.rkt
@@ -0,0 +1,106 @@
+#lang racket
+
+(require "utils.rkt")
+
+
+; section 2.4
+(define (add-complex z1 z2)
+  (make-from-real-imag (+ (real-part z1) (real-part z2))
+                       (+ (imag-part z1) (imag-part z2))))
+
+(define (sub-complex z1 z2)
+  (make-from-real-imag (- (real-part z1) (real-part z2))
+                       (- (imag-part z1) (imag-part z2))))
+
+(define (mul-complex z1 z2)
+  (make-from-mag-ang (* (magnitude z1) (magnitude z2))
+                     (+ (angle z1) (angle z2))))
+
+(define (div-complex z1 z2)
+  (make-from-mag-ang (/ (magnitude z1) (magnitude z2))
+                     (- (angle z1) (angle z2))))
+
+;;; constructors and selectors for rectangular form
+(define (real-part-rect z) (car z))
+(define (imag-part-rect z) (cdr z))
+
+(define (magnitude-rect z)
+  (sqrt (+ (square (real-part-rect z))
+           (square (imag-part-rect z)))))
+
+(define (angle-rect z)
+  (atan (imag-part-rect z) (real-part-rect z)))
+
+(define (make-from-real-imag-rect x y) (attach-tag 'rectangular (cons x y)))
+
+(define (make-from-mag-ang-rect r a)
+  (attach-tag 'rectangular (cons (* r (cos a)) (* r (sin a)))))
+
+;;; constructors and selectors for polar form
+(define (real-part-polar z) (* (magnitude-polar z) 
+                               (cos (angle-polar z))))
+
+(define (imag-part-polar z) (* (magnitude-polar z) 
+                               (sin (angle-polar z))))
+
+(define (magnitude-polar z) (car z))
+(define (angle-polar z)     (cdr z))
+
+(define (make-from-real-imag x y)
+  (attach-tag 'polar
+              (cons (sqrt (+ (square x) (square y)))
+                    (atan y x))))
+
+(define (make-from-mag-ang r a) (attach-tag 'polar (cons r a)))
+
+;; type tag
+(define (attach-tag type-tag contents)
+  (cons type-tag contents))
+
+(define (type-tag datum)
+  (if (pair? datum)
+      (car datum)
+      (error "Bad tagged datum -- TYPE-TAG" datum)))
+
+(define (contents datum)
+  (if (pair? datum)
+      (cdr datum)
+      (error "Bad tagged datum -- CONTENTS" datum)))
+
+(define (rectangular? z)
+  (eq? (type-tag z) 'rectangular))
+
+(define (polar? z)
+  (eq? (type-tag z) 'polar))
+
+;; generic procedures
+(define (real-part z)
+  (cond
+    [(rectangular? z) (real-part-rect (contents z))]
+    [(polar? z)       (real-part-polar (contents z))]
+    [else (error "unknown type -- real-part" z)]))
+
+(define (imag-part z)
+  (cond
+    [(rectangular? z) (imag-part-rect (contents z))]
+    [(polar? z)       (imag-part-rect (contents z))]
+    [else (error "unknown type -- imag part" z)]))
+
+(define (magnitude z)
+  (cond
+    [(rectangular? z) (magnitude-rect (contents z))]
+    [(polar? z)       (magnitude-polar (contents z))]
+    [else (error "Unknown type -- MAGNITUDE" z)]))
+
+(define (angle z)
+  (cond [(rectangular? z) (angle-rect (contents z))]
+        [(polar? z)       (angle-polar (contents z))]
+        [else (error "Unknown type -- ANGLE" z)]))
+
+(define (make-from-real-imag x y)
+  (make-from-real-imag-rect x y))
+
+(define (make-from-mag-ang r a)
+  (make-from-mag-ang-polar r a))
+
+;;; data directed programming