solution to 4.41 and 4.42

diff --git a/src/sicp/ex4_41.rkt b/src/sicp/ex4_41.rkt
new file mode 100644 (file)
index 0000000..ba4b97b
--- /dev/null
+++ b/src/sicp/ex4_41.rkt
@@ -0,0 +1,24 @@
+#lang racket
+(require "distinct.rkt")
+
+(define (multiple-dwelling)
+  (for/list ([baker '(1 2 3 4 5)]
+             #:unless (= baker 5)
+             [cooper '(1 2 3 4 5)]
+             #:unless (= cooper 1)
+             [fletcher '(1 2 3 4 5)]
+             #:unless (or (= fletcher 1)
+                          (= fletcher 5))
+             [miller '(3 4 5)]
+             #:when (> miller cooper)
+             [smith '(1 2 3 4 5)]
+             #:when (and (not (= (abs (- smith fletcher)) 1))
+                         (not (= (abs (- fletcher cooper)) 1))
+                         (distinct? (list baker cooper fletcher miller smith))))
+    (list (list 'baker baker)
+          (list 'cooper cooper)
+          (list 'fletcher fletcher)
+          (list 'miller miller)
+          (list 'smith smith))))
+
+(multiple-dwelling)
\ No newline at end of file

diff --git a/src/sicp/ex4_42.rkt b/src/sicp/ex4_42.rkt
new file mode 100644 (file)
index 0000000..76ecad5
--- /dev/null
+++ b/src/sicp/ex4_42.rkt
@@ -0,0 +1,31 @@
+#lang racket
+
+(require "amb-eli.rkt")
+(require "distinct.rkt")
+
+(define (xor x y)
+  (cond [(and x y) #f]
+        [(and (not x) (not y)) #f]
+        [else #t]))
+
+(define (liars)
+  (let ([betty (amb 1 2 3 4 5)]
+        [ethel (amb 1 2 3 4 5)]
+        [joan  (amb 1 2 3 4 5)]
+        [kitty (amb 1 2 3 4 5)]
+        [mary  (amb 1 2 3 4 5)])
+    (assert (distinct? (list betty ethel joan kitty mary)))
+    (assert (xor (= kitty 2) (= betty 3)))
+    (assert (xor (= ethel 1) (= joan 2)))
+    (assert (xor (= joan 3)  (= ethel 5)))
+    (assert (xor (= kitty 2) (= mary 4)))
+    (assert (xor (= mary 4)  (= betty 1)))
+    (list (list 'betty betty)
+          (list 'ethel ethel)
+          (list 'joan joan)
+          (list 'kitty kitty)
+          (list 'mary mary))))
+
+(collect (liars))
+
+;; '(((betty 3) (ethel 5) (joan 2) (kitty 1) (mary 4)))