vd02: accumulator, parent's house wizard name.

10-accumulators/vd02-wizard-parent-house.rkt

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+#lang htdp/asl
+(require 2htdp/image)
+
+;
+; PROBLEM:
+;
+; In the Harry Potter movies, it is very important which of the four houses a
+; wizard is placed in when they are at Hogwarts. This is so important that in
+; most families multiple generations of wizards are all placed in the same family.
+;
+; Design a representation of wizard family trees that includes, for each wizard,
+; their name, the house they were placed in at Hogwarts and their children. We
+; encourage you to get real information for wizard families from:
+;    http://harrypotter.wikia.com/wiki/Main_Page
+;
+; The reason we do this is that designing programs often involves collection
+; domain information from a variety of sources and representing it in the program
+; as constants of some form. So this problem illustrates a fairly common scenario.
+;
+; That said, for reasons having to do entirely with making things fit on the
+; screen in later videos, we are going to use the following wizard family tree,
+; in which wizards and houses both have 1 letter names. (Sigh)
+
+
+(define-struct wiz (name house kids))
+;; Wizard is (make-wiz String String (listof Wizard))
+;; Interp: a Wizard with name, house and list of children.
+
+(define Wa (make-wiz "A" "S" empty))
+(define Wb (make-wiz "B" "G" empty))
+(define Wc (make-wiz "C" "R" empty))
+(define Wd (make-wiz "D" "H" empty))
+(define We (make-wiz "E" "R" empty))
+(define Wf (make-wiz "F" "R" (list Wb)))
+(define Wg (make-wiz "G" "S" (list Wa)))
+(define Wh (make-wiz "H" "S" (list Wc Wd)))
+(define Wi (make-wiz "I" "H" empty))
+(define Wj (make-wiz "J" "R" (list We Wf Wg)))
+(define Wk (make-wiz "K" "G" (list Wh Wi Wj)))
+
+(define IMG1-FAMILY-TREE (bitmap "../imgs/028-wizzard-family-tree-houses.jpg"))
+
+; Template, arb-arity-tree, encapsulated with local.
+(define (fn-for-wiz w)
+  (local [(define (fn-for-wiz)
+            (... (wiz-name w)
+                 (wiz-house w)
+                 (fn-for-low (wiz-kids w))))
+          (define (fn-for-low low)
+            (cond [(empty? low) (...)]
+                  [else
+                   (... (fn-for-wiz (car low))
+                        (fn-for-low (cdr low)))]))]
+    (fn-for-wiz w)))
+
+;; I have to keep the parent's house in the lost context accumulator.
+;; It is not that every children have to be in the same house as the
+;; initial node to be listed, they have to be in the same house as their
+;; immediate ancestor.
+;;
+;; Wizard E is not in the house G, but it is in the same house R as its
+;; parent J, therefore, starting at Kg, E should be in the resulting list.
+;; The same for F, which is also in the same house R as F's parent, and
+;; A is in the same house as its parent G.
+
+;; Wizard -> (listof String)
+;; Produces a list of names of every wizard in the tree that was placed
+;; in the same house as their immediate parent.
+(check-expect (names We) empty)
+(check-expect (names Wh) empty)
+(check-expect (names Wg) (list "A"))
+(check-expect (names Wk) (list "E" "F" "A"))
+
+;; `ph` is an accumulator.
+
+;; Template from Wizard plus lost context acc called ph (parent-house).
+(define (names w)
+  ;; ph is String;
+  ;; The house of this wizard's immediate parent.
+  ;; Root tree doesn't have a parent, so we just use "" for the ph.
+  ;; (names Wk)
+  ;; (fn-for-wiz Wk "")
+  ;; (fn-for-wiz Wh "G")
+  ;; (fn-for-wiz Wc "S")
+  ;; (fn-for-wiz Wd "s")
+  ;; (fn-for-wiz Wi "G")
+  (local [;; Wizard String -> (listof String)
+          (define (fn-for-wiz w ph)
+            (if (string=? ph (wiz-house w))
+                (cons (wiz-name w)
+                      (fn-for-low (wiz-kids w) (wiz-house w)))
+                (fn-for-low (wiz-kids w)
+                            (wiz-house w))))
+
+          ;; (listof Wizard) String -> (listof String)
+          (define (fn-for-low low ph)
+            (cond [(empty? low) empty]
+                  [else
+                   (append (fn-for-wiz (car low) ph)
+                           (fn-for-low (cdr low) ph))]))]
+    (fn-for-wiz w "")))
+