PROLOG Programming   
 

      University of Wales, Cardiff   

 

        January 1997   

 

        Omer F. Rana   
        omer@cs.cf.ac.uk   
        Room S/2.03   

 

  What is Prolog and why should I bother ? 

  
   PROLOG  : PRogramming in LOGic  

  
  5th generation computer language 
  Used in programming for artificial intelligence 
  based on a subset of predicate calculus (Horn clauses)
  The    Edinburgh  syntax
 

A    DIFFERENT  programming style from    PASCAL  or
   C   
 [1in] 6in  0.01cm 

 

A language for :  
   ARTIFICIAL INTELLIGENCE ,  
 
with numerous
application domains (such as)    DATABASES,  
NATURAL LANGUAGE PROCESSING,  
IMAGE ANALYSIS,  
DECISION SUPPORT SYSTEMS,  
FINANCIAL MODELS, INTERNET APPLICATIONS  and many others ...  
 
  
   Understanding the experts   
   Intelligent Reasoning   
 
  Books n' stuff 
 
  Prolog Programming for Artificial Intelligence    Bratko  Addison-Wesley, 90
  Programming in Prolog,    Clocksin and Mellish  Berlin : Springer-Verlag
      87
  The art of Prolog : advanced programming techniques,    Sterling and Shapiro  MIT P
ress, 94
  Problem solving with Prolog,    Stobo  London : Pitman 89
  LPA MacProlog32 (technical manuals)    Logic Programming Associates 
 

   However, for an introductory course, any book on Prolog
is fine.   

  Course Outline 

  Declarative programming style
  Translating simple problems into    Prolog 
  Language Syntax
  Computing with    Prolog 
  Programming techniques and styles
  Understanding execution of    Prolog  programs
  User interfaces in    MacProlog 
  Stand alone applications and link with other
programming languages 
  Applications :    Databases  and the    Internet 
 
   Lab Handout for    LPA MacProlog  
 

     LPA  MacProlog32 
 
  Will be used for programming in the labs
  On a (surprise surprise) Macintosh
  See    User Guide  for details
  Mail questions to me at    omer@cs.cf.ac.uk 
     Friday-hour  for support
     Programming   Practise 
  Lots of chance to    play  with    Prolog , also
compiler available on the internet for PC.
  Emphasis on    Logic Programming 

  Comparison between Pascal and Prolog 

  PASCAL 
  
  is    compiled  to an executable file. 
  is    imperative  - consists of a sequence of instructions to execute.
  program flow composed of    conditionals  and    loops .
  numerous    built-in  functions (language grammar).
  an expression can evaluate to one of    many  values.
 

  PROLOG 
 
  is    interpreted .
  is    declarative  - contains a series of    definitions  which are 
accessed as required (in any order) by the program during execution.
  the    HOW  of solving the problem is    NOT  defined, only what is 
   TRUE or NOT  of a certain state of affairs is described.
  a statement can either be    TRUE  or    FALSE 
  a few    built-in  rules, and a few    fundamental operations .
 
   PASCAL  programs contain a series of    INSTRUCTIONS 
given to the computer as    PROCEDURES  to perform. The order
of doing things is    SPECIFIED  and a number of    ACTIONS 
performed as a consequence.  


   PROLOG  programs contain a set of    FACTS  and    RULES .
Aspects of the problem which are    TRUE  or    FALSE  are described
(defined), rather than the method to achieve a    RESULT .  

   For instance :   
 
	likes(omer, donuts) 
	likes(paul, neighbours)

  Example   

   In predicate calculus  
 
	green(fonzie)
	green(puff)
	green(martian)

	dragon(godzilla)
	dragon(puff)
 

The corresponding translation in    Prolog  will be :  

fly(X) :-	
	green(X),
	dragon(X). 	
 
body      antecedents  that must be true.

A Logical Deduction is called a    Query   

 
     Asking for Confirmation   
 
Q : fly(puff)  
A : yes  
 
Q : fly(fonzie)  
A : no  
 
Q : fly(kermit)  
A : no (   no   unproven, not false   
 
     Asking for values   
 
Q : fly(X)  
A : X = puff  
 
Q : dragon(X)  
A : X = godzilla  
 X = puff  
 

Find a fact to match the    goal  :    Binding 

Some    Prolog Facts :   

Find a fact to match the    goal  :    Binding   
 
So in the previous example, for instance,    puff 
was found to be a valid answer. If more than one possible
answers existed to the problem, then the first    goal  
variable is    unbound  (i.e. its value discarded) and the
next matching value    bound  to it.  
 
 
     Matching  of goals to facts
     Searching  for solutions

Fundamental processes in the understanding of how    Prolog 
works.  

  Another Example 

   In predicate calculus  :
 
now pick a few students :  
  

	student(patrick) student(jane) 
	student(gwyn) student(ahmed)
	student(helen) student(vijay)
 

those with money left / free time :  
  
	money-left(gwyn)	free-time(ahmed)	
	money-left(jane)	free-time(jane)
	money-left(helen)	free-time(vijay)

   Translation Into Prolog  :  

holiday(X) :-
	student(X),
	money_left(X).

party(X) :-
	student(X),
	free_time(X). 

lucky(X) :-
	student(X), money_left(X) ;
	student(X), free_time(X).

student(patrick).
student(jane).
student(gwyn).
student(ahmed).
student(helen). 
student(vijay).
money_left(gwyn). 
free_time(ahmed).        
money_left(jane).        
free_time(jane).
money_left(helen).       
free_time(vijay).
 

Things to note :  
 
     Layout  : Keep it readable.
     Spaces, Hyphens and Underscores .
     Full stops and commas .
     The Universe of discourse  (limited to that defined by the program)
 
  Prolog Syntax 

  Follows the Edinburg syntax  
 
/*	one line comment */  
 
/* 	Multi  
	line comment */ 
    the rest of this line is a comment
     Terms  are the building blocks of a program
  for instance,    Numbers   
 
   Integers : 12 0 -24   
 
   Floating Point : 2.5 -0.34 10e-4 

A    query  can be used to determine or decide upon
number syntax, so for instance :  
 
Q :	integer(12)  
A :	yes  
Q :	float(10.2)  
A :	yes  
Q : 	integer(-7.43)  
A :	no  
Q :	number(-2.34)  
A :	yes  
Q :	number(5322)  
A :	yes  
 
 
     integer ,    float  and    number  are built in
predicates.
     dragon ,    green  and    fly  are user defined
predicates.
 
   Atoms   

Constants that start with a lower case letter, followed by zero
or more alphanumeric characters, for instance :  
 
donut  
can dance  
fortran90  
 
symbolic (sequence of symbolic characters)   
 
++  
    
 
and those with    special meaning  in Prolog :  
 
! (exclamation mark)  
, (coma)  
; (semi-colon)  

quotes (any sequence of characters in single quotes)  
 
'Apple'  
'Alia'  
' them'  
 
again a query format can be used to deduce atoms  
 
Q : 	atom(alia)  
A :	yes  
Q :	atom(123)  
A :	no  
Q :	atomic(123)  
A :	yes	(   because is an integer )  
Q :	atomic('123')  
A :	yes	(   because is an atom )  
 
What are the distinctions ?  

   Variables   
 
alphanumeric sequence of characters (include  ),
starting with a capital letter or underscore, such as  
 
X  
Apple  
 name  
Failed student  
 
So a query such as :  
 
Q : var(Apple)  
A : yes  
Q : nonvar( name)  
A : no  
Q : var(ola)  
A : no  
Q : var('Apple')  
A : no 	(   because is an atom )  
 
Variables can be anonymous, denoted by the  
  (underscore alone), and used when a return  
value is not needed  
 
for instance,  
 
   born(heather,cardiff)  
born(ben, prague)   
 
Q :	born( ,cardiff)  
A :	yes  
 
The question being asked is :  
 
   Is anybody born in cardiff ? , but I    do not care 
who !  
 
   Note  :     Do not use  2 or  (number) as a variable name,
as this is how MacProlog32 uses anonymous variables, and represents
them internally. 

  Scope of a variable 
 

Within a clause, all occurrences of an identical variable refer
to the same value.  
 
For instance :  
 
   fly(X) :- green(X),  dragon(X).   
 
and    happy(X) :- pass exam(X,Y).   
 
here,    X  in    fly(X)  and    happy(X)  are
not related.  
 
However, the occurrance of    X  in the definition of
   fly(X)  refer to the same thing.  
 
Also, note the change in    arity  of the clause
   happy(X) .  

 

There are some    exceptions  however for such
   anonymous  variables :  
 
   balanced child(X) :-   
      brother(X, ),   
      sister(X, ).   
 
Here, the variable represented by   refers to
different people  
 
and would be represented    internally  as :  
 
   balanced child(X) :-   
      brother(X, 483),   
      sister(X, 485).   
 
   The advanced    Trace  mechanism can be used
to show internal variable names   

Consider the program :  

check_number(0). 

check_number(N) :-
	N < 0,
	N1 is N+2,
	check_number(N1).

check_number(N) :-
	N > 0,
	N1 is N-2,
	check_number(N1).
 
What happens when we give the queries :  
 
   check number(4)  
check number(100)  
check number(15)  
check number(125)   

Query :  

Q : check number(4)  
A : yes  

   The program goes into a loop, nothing appears to be happening
on the screen. The loop will only stop until there is no more memory
left for evaluation. Hence, the program will have to be interrupted.   
 
   What can we do to prevent this happening ?   
 
   check number(1).    
 
Q : check number(15)  
A : yes  
 
   the new information also has to be added at a specific point,
because of the way evaluation is performed by Prolog 

  Compound Terms 

Atom (functor) followed by k   1 terms, enclosed in
brackets and seperated by commas  
 
   k  denotes the    arity   
 
e.g. student(ursula)  
   composer(bach,baroque)  
   disk jockey(X,party(X))  
 
   tuples  : sequences of terms, enclosed in brackets and
separated by commas  
 
e.g. representing a date :  
   23, march, 97)  
as, is an atom, the tuple can be seen as :  
','(10,','(march, 97))    (internal handling)   
 
Q : compound( foo(a,B,32))  
A : yes  
Q : compound( ' Others'( Names ))  
A : yes  
Q : compound(123)  
A : no