added today's prolog file

Frederik/README.md

@@ -7,3 +7,4 @@ AI 1 - 2019/2020
 * 2019-10-28: `prolog/2019-10-28-tutorial.pl` (simple prolog exercises)
 * 2019-10-31 (substition for Katja): `prolog/2019-10-31-tutorial.pl` (simple prolog exercises)
 * 2019-11-04: `prolog/2019-11-04-trees.pl` (some simple tree exercises)
+* 2019-11-11: `2019-11-11-nary-trees.pl` (tree exercise in preparation for tree-search homework)

Frederik/prolog/2019-11-11-nary-trees.pl

+% The tree definition from the homework assignment.
+istree(tree(Value,Children)) :-
+    string(Value),subtrees(Children).
+subtrees([]).
+subtrees([(Cost,T)|Rest]) :-
+    number(Cost),istree(T), subtrees(Rest).
+
+
+% An example tree.
+% If you want to call e.g. addweights with this tree, you can do it with the following trick:
+% tree1(X), addweights(X, Sum).
+tree1(tree("A", [(2, tree("B", [(1, tree("E", [])), (2, tree("F", []))])),
+                 (5, tree("C", [(2, tree("G", []))])),
+                 (2, tree("D", [(1, tree("X", [(1, tree("Y", []))]))]))])).
+
+% Add up all the weights of the edges in the tree.
+% (we called it cw in the tutorial)
+addweights(tree(_, []), 0).
+addweights(tree(_, [(X, T)|Rest]), S) :-
+   addweights(T, S1),
+   addweights(tree("asdf", Rest), S2),
+   S is X + S1 + S2.
+
+% Also adds up all the weights, but in a different way:
+% In the previous attempt, we always expected a tree as argument and constructed trees on demand.
+% In this approach we'll take a list of (Cost, Tree) pairs instead.
+% This is not as pretty, but a useful approach for some tree search algorithms.
+addweights2([], 0).
+addweights2([(X, tree(_, Children))|Rest], S) :-
+    addweights2(Rest, S1),
+    addweights2(Children, S2),
+    S is S1 + S2 + X.
+addweights2(T, S) :-
+    istree(T),
+    addweights2([(0, T)], S).