changeset 61:49cb2f7af872

Fix the Day 1 collaboration problems
author IBBoard <dev@ibboard.co.uk>
date Fri, 29 Sep 2017 20:00:50 +0100
parents 643eec1f9e3a
children 96e77f914f9b
files 3-Prolog/day1-self-study.pl
diffstat 1 files changed, 9 insertions(+), 3 deletions(-) [+]
line wrap: on
line diff
--- a/3-Prolog/day1-self-study.pl	Wed Sep 27 20:48:28 2017 +0100
+++ b/3-Prolog/day1-self-study.pl	Fri Sep 29 20:00:50 2017 +0100
@@ -21,8 +21,8 @@
 wrote(jack_yeovil, route_666).
 wrote(jack_yeovil, krokodil_tears).
 
-collaborated(Author1, Author2, Book) :- \+(Author1 = Author2), author(Author1), author(Author2),
-    wrote(Author1, Book), wrote(Author2, Book), book(Book).
+collaborated(Author1, Author2, Book) :- author(Author1), author(Author2),
+    wrote(Author1, Book), wrote(Author2, Book), book(Book), Author1 \== Author2.
 
 collaborated(Author1, Author2) :- collaborated(Author1, Author2, Book).
 % collaborated(terry_pratchett, stephen_baxter). returns true, but collaborated(Author1, Author2).  just says "no", as does
@@ -35,4 +35,10 @@
 % wrote(terry_pratchett, Book). works to return all books by an author, though.
 %
 % However, having added a three-argument version, it can correctly identify the books that someone collaborated on if
-% you supply both authors but it won't supply an author when given one author and the book title.
\ No newline at end of file
+% you supply both authors but it won't supply an author when given one author and the book title.
+%
+% On tracing this and the W&G example then it seems that "\+(…)" doesn't play nicely with variables. The W&G examples
+% worked because we only asked "are these two people friends" questions, not "who is friends with" questions.
+%
+% What we actually need is "\==" (not equal/equivalent). Also, we need to put it at the end so that Author1 and Author2 have
+% been assigned by that point.
\ No newline at end of file