The Story So Far ... 

  Explanation in logic. 
  Definitions and facts.
  Predicates / Rules.
  Translation from a logic construction.               
  Conjunction and Disjunstion.
  Nested rules.
  Rule Syntax (Anatomy of a predicate).
  Atoms, Variables.
  Anonymous Variables and PlaceHolders.
  Yes/No Queries.
  Search-a-database Query.
 
   Any problems with these ?  

  Programming Guidelines 
 
  Meaningful names for variables and predicates.
  simple predicates.
  keep database seperate.
  Put plenty of comments.
  Do not use variable names like  (number)
 

  More Examples 

Two people are neighbours if they live on the same street
but in different houses. 

 neighbour( Person_A, Person_B) :-
   address( Person_A, Street, Num),
   address( Person_B, Street, Num1).

Translate the following into Prolog rules :  
 
  Nothing is good and bad at the same time.
  X lives if X is a human, if he/she breathes and if his/her pulse-rate
is greater than zero.
  I hate McDonalds.
  Liam plays all the songs that the Beatles played without a keyboard.
  At the restaurant I can have cheese with apple or bread or coffee 
and chocolate with donuts.
 
  A program ... FINALLY!! 

/* hifi(Name, Rms, Tapes, Cds, Turntable, Price) */
hifi(sony, 28, 2,1,1,279).
hifi(aiwa, 25, 2,1,1,249).
hifi(sanyo,12, 2,3,1,219).
hifi(pioneer,40,2,1,0,379).
hifi(akai, 12, 2,3,1,379).

preferable( Hifi, Price) :-
 hifi(Name, Rms, Tapes, Cds, Turntable, Price),
 Rms > 25.
preferable( Hifi, Price) :-
 hifi(Name, Rms, Tapes, Cds, Turntable, Price),
 Tapes > 1.

will_do( Hifi, Price) :-
 hifi(Name, Rms, Tapes, Cds, Turntable, Price),
 Tapes > 0.
will_do( Hifi, Price) :-
 hifi(Name, Rms, Tapes, Cds, Turntable, Price),
 Rms > 10.

Program is constructed from :  
 
   Procedures  : set of clauses about the same relation,
containing the same predicate in their heads.  
 
   Price  is a shared variable - restricts range. 
 
   Procedures are  :  
   hifi/6   
   preferable/2   
   will do/2   
 
   The Program constitutes the    formal definition  of
the problem, it is not yet the solution. The solution is 
what occurs when we ask Prolog a question.   
 
   preferable( Hifi, Price), Price < 200.   
   no   
 
   preferable( Hifi, Price), Price < 300.   
   Hifi=sony, Price=279   
   Hifi=akai, Price=279)  

  Compound Terms and Structures   
 
 
  Most real world objects constitute a combination of items
or atoms put together.
  To denote a structure, we chose a    functor  and 
its    arguments .
 
An example, a menu :  
 
lunch( Soft drink, Main meal, Donut, Beverage)  
 
eg : 
lunch( diet coke, pizza, donut, coffee)  
 
donut(jam donut, ring donut, choc donut, caramel donut, custard donut)  
 
With lunch now becoming a mouthful :  
 
lunch( Soft drink, Main meal,  
 donut(jam donut, ring donut, choc donut, caramel donut, custard donut),  
 Beverage).  
 
etc ...  
 
As structures get more complex, it is usually better to start
representing them as    Trees , so that the relationships between
sub-structures become more clear.  

  More Lists  

A non-empty list consists of a    HEAD  and a    TAIL .  
 
The    FIRST ELEMENT  is said to be the    HEAD  of the list
and the rest constitutes the    TAIL .  
 
We can make this explicit, by using the   notation, for instance :

[1,2,3] = [1   [2,3]]


["One", "Two", "Three"]		"One" 	["Two","Three"]


[145]				145	[]


[]				undefined undefined


[[1,2],[3,4,5],[]]		[1,2]	[[3,4,5],[]]


[a]				a	[]

   List is a very important and useful data structure within
Prolog, and extensively used. 

  Equating Lists 

List1	:	[X,Y,Z]
List2	:	[4,5,6]

List1 = List2	X=4, Y=5, Z=6

List1	:	["hello"]
List2	:	[X Y]

List1 = List2 	X = "hello", Y=[]

List1	:	[1,2,3,4]
List2 	:	[X,Y Z]

List1 = List2	X=1, Y=2, Z=[3,4]

List1	:	[1,X]
List2	:	[Y,3]

List1 = List2	Y=1, X=3

  List Membership   

  
member(X,L) 
member(b,[a,b,c]). 

One : 
member(X,[X   Tail]).

member(X, [Head   Tail]) :-
 member(X, Tail). 

Two :
member(X,[X _]).

member(X, [_ Tail]) :-
 member(X,Tail).
 

member(2,[1,4,2,3,5]).  
 
   Ist Clause  : No match  
   2nd Clause  : Member if member of [4,2,3,5]  
   Ist Clause  : Satisfied  
   Hence Answer = yes   

  Alternate List Representation 

 
  Possible to represent lists like other Prolog structures
using a special functor '.' (dot).
     .(Head, Tail) 
  For instance, a list like :  
 
[miles, evans, holliday, parker, getz]  
 
can be expressed as :  
 
.(miles, .(evans, .(holliday, .(parker, .(getz,[])))))

 The  dot notation for internal representation

List = [ a, b, c, d   Tail ]

then extend Tail to 
Tail = [ e   Tail1 ] 

List = [a, b,c,d,e   Tail1]

Hence, arbitary extension of lists.