Solutions to 3.73, 3.74, 3.75, 3.76, 3.77, 3.78, 3.79, 3.80 and 3.81

diff --git a/src/sicp/ex3_73.rkt b/src/sicp/ex3_73.rkt
new file mode 100644 (file)
index 0000000..ff0abd3
--- /dev/null
+++ b/src/sicp/ex3_73.rkt
@@ -0,0 +1,9 @@
+#lang racket
+
+(define (RC R C dt)
+  (define (RC1 i initial-vc)
+    (add-streams (integral (scale-stream i (/ 1.0 C)) initial-vc dt)
+                 (scale-stream i R)))
+  RC1)
+
+(define RC1 (RC 5 1 0.5))
\ No newline at end of file

diff --git a/src/sicp/ex3_74.rkt b/src/sicp/ex3_74.rkt
new file mode 100644 (file)
index 0000000..9d5bb78
--- /dev/null
+++ b/src/sicp/ex3_74.rkt
@@ -0,0 +1,21 @@
+#lang racket
+
+(define (make-zero-crossings input-stream last-value)
+  (cons-stream
+   (sign-change-detector (stream-car input-stream) last-value)
+   (make-zero-crossings (stream-cdr input-stream)
+                        (stream-car input-stream))))
+
+(define (sign-change-detector current-value previous-value)
+  (cond [(and (positive? previous-value)
+              (positive? current-value)) 0]
+        [(and (positive? previous-value)
+              (negative? current-value)) -1]
+        [(and (negative? previous-value?)
+              (positive? current-value)) +1]
+        [else 0]))
+
+(define zero-crossings (make-zero-crossings sense-data 0))
+
+;; eva's implementation
+(define zc (stream-map sign-change-detector sense-data (cons-stream 0 sense-data)))

diff --git a/src/sicp/ex3_75.rkt b/src/sicp/ex3_75.rkt
new file mode 100644 (file)
index 0000000..f569b32
--- /dev/null
+++ b/src/sicp/ex3_75.rkt
@@ -0,0 +1,12 @@
+#lang racket
+
+#|
+
+The procedure find average of the current sensor value with the last average value. This
+is wrong. the symbol 'last-value' is used differently in different places. In the average
+calculation, this should be the last sensor value. In the sign-change-detector this should
+be last average value.
+
+Solution is for the make-zero-crossing to accept the last-average-value as another parameter.
+
+|#
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diff --git a/src/sicp/ex3_76.rkt b/src/sicp/ex3_76.rkt
new file mode 100644 (file)
index 0000000..f4f6fe1
--- /dev/null
+++ b/src/sicp/ex3_76.rkt
@@ -0,0 +1,17 @@
+#lang racket
+
+(define (smooth stream)
+  (cons-stream (/ (+ (stream-car stream)
+                     (stream-car (stream-cdr stream))) 
+                  2.0)
+               (smooth (stream-cdr stream))))
+
+(define (make-zero-crossings input-stream last-value)
+  (cons-stream
+   (sign-change-detector (stream-car input-stream) last-value)
+   (make-zero-crossings (stream-cdr input-stream)
+                        (stream-car input-stream))))
+
+(define zero-crossings (make-zero-crossings (smooth sense-data) 0))
+               
+               
\ No newline at end of file

diff --git a/src/sicp/ex3_77.rkt b/src/sicp/ex3_77.rkt
new file mode 100644 (file)
index 0000000..c7de078
--- /dev/null
+++ b/src/sicp/ex3_77.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+(define (integral integrand delayed-initial-value dt)
+  (cons-stream initial-value
+               (let ((integrand (force delayed-initial-value)))
+                 (if (stream-null? integrand)
+                     the-empty-stream
+                     (integral (stream-cdr integrand)
+                               (+ (* dt (stream-car integrand))
+                                  initial-value)
+                               dt)))))
\ No newline at end of file

diff --git a/src/sicp/ex3_78.rkt b/src/sicp/ex3_78.rkt
new file mode 100644 (file)
index 0000000..3a1282a
--- /dev/null
+++ b/src/sicp/ex3_78.rkt
@@ -0,0 +1,9 @@
+#lang racket
+
+(define (solve-2nd a b y0 dy0 dt)
+  (define y (integral (delay dy) y0 dt))
+  (define dy (integral (delay ddy) dy0 dt))
+  (define ddy (add-streams (scale-stream dy a)
+                           (scale-stream y b)))
+  y)
+

diff --git a/src/sicp/ex3_79.rkt b/src/sicp/ex3_79.rkt
new file mode 100644 (file)
index 0000000..4fa6093
--- /dev/null
+++ b/src/sicp/ex3_79.rkt
@@ -0,0 +1,7 @@
+#lang racket
+
+(define (solve-2nd f y0 dy0 dt)
+  (define y (integral (delay dy) y0 dt))
+  (define dy (integral (delay ddy) dy0 dt))
+  (define ddy (stream-map f dy y))
+  y)
\ No newline at end of file

diff --git a/src/sicp/ex3_80.rkt b/src/sicp/ex3_80.rkt
new file mode 100644 (file)
index 0000000..989c6d7
--- /dev/null
+++ b/src/sicp/ex3_80.rkt
@@ -0,0 +1,12 @@
+#lang racket
+
+(define (RLC R L C dt)
+  (define (rlc1 vc0 il0)
+    (define vc (integral (delay dvc) vc0 dt))
+    (define dvc0 (scale-stream il (/ -1 C)))
+    (define il (integral (delay dil) il0 dt))
+    (define dil (add-streams (scale-stream vc (/ 1 L))
+                             (scale-stream il (/ -R L))))
+    (stream-map (lambda (x y) (cons x y)) vc il)))
+
+

diff --git a/src/sicp/ex3_81.rkt b/src/sicp/ex3_81.rkt
new file mode 100644 (file)
index 0000000..158bec4
--- /dev/null
+++ b/src/sicp/ex3_81.rkt
@@ -0,0 +1,17 @@
+#lang racket
+
+(define (rand-seq init req-stream)
+  (let ((req (stream-car req-stream)))
+    (cond ((eq? req 'reset)
+           (let ((new-init (stream-car (stream-cdr req-stream))))
+             (let ((new-rand-val (rand-update new-init)))
+               (cons-stream
+                new-rand-val
+                (rand-seq new-rand-val 
+                          (stream-cdr (stream-cdr req-stream)))))))
+          ((eq? req 'generate)
+           (let ((new-val (rand-update init)))
+             (cons-stream 
+              new-val
+              (rand-seq new-val (stream-cdr req-stream))))))))
+          
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