solutions to 4.38, 4.39 and 4.40

[sicp.git] / src / sicp / ex4_40.rkt

diff --git a/src/sicp/ex4_40.rkt b/src/sicp/ex4_40.rkt
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+#lang racket
+
+#|
+
+how many sets of assignments are there of people to floors, both before and after the requirement that floor assignments be distinct? 
+
+Before - 5 * 5 * 5 * 5 * 5 = 3125
+After  - 5 * 4 * 3 * 2 * 1 = 120
+
+|#
+
+(require "amb-eli.rkt")
+(require "distinct.rkt")
+
+(define (multiple-dwelling-1)
+  (let ([baker (amb 1 2 3 4)]
+        [cooper (amb 2 3 4 5)]
+        [fletcher (amb 2 3 4)]
+        [miller (amb 3 4 5)]
+        [smith (amb 1 2 3 4 5)])
+    ;(assert (not (= baker 5)))
+    ;(assert (not (= cooper 1)))
+    ;(assert (not (= fletcher 5)))
+    ;(assert (not (= fletcher 1)))
+    (assert (> miller cooper))
+    (assert (not (= (abs (- smith fletcher)) 1)))
+    (assert (not (= (abs (- fletcher cooper)) 1)))
+    (assert (distinct? (list baker cooper fletcher miller smith)))
+    (list (list 'baker baker)
+          (list 'cooper cooper)
+          (list 'fletcher fletcher)
+          (list 'miller miller)
+          (list 'smith smith))))
+
+(collect (multiple-dwelling-1))
+
+;; another implementation
+(define (multiple-dwelling-2)
+  (let ([cooper (amb 2 3 4 5)]
+        [miller (amb 3 4 5)])
+    (assert (> miller cooper))
+    (let ([fletcher (amb 2 3 4)])
+      (assert (not (= (abs (- fletcher cooper)) 1)))
+      (let ([smith (amb 1 2 3 4 5)])
+        (assert (not (= (abs (- smith fletcher)) 1)))
+        (let ([baker (amb 1 2 3 4)])
+          (assert (distinct? (list baker cooper fletcher miller smith)))
+          (list (list 'baker baker)
+                (list 'cooper cooper)
+                (list 'fletcher fletcher)
+                (list 'miller miller)
+                (list 'smith smith)))))))
+
+(collect (multiple-dwelling-2))
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