Bab 4 Prolog

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PROLOG

Andrian RakhmatsyahInstitut Teknologi Telkom



PROLOG is a programming system in whichlogic is used as a programming languageas well as a framework for programinterpretation

Generally, a PROLOG program is desciption

of a world (that is a collection of objectthat are related to each other in some

way), written in the language of PROLOG.



PROLOG can be used either as a

Software Tool for development of intelligent

systems (such as expert systems and robotcontrol systems) or

Simply as a general-purpose programming

language with a powerful mechanism for

problem solving.



Basically, the system works as follows : Users submits a description of a problem

written in PROLOG language.

The PROLOG interpreter then applies logicalreasoning to find answers for the problem



Conventional Programming• The programmer instruct the machine HOW to solve a problem

by performing a given sequence of actions

• The machine carries out the instructions in a specified order

Logic Programming

• The programmer defines WHAT the problem is, using thelanguage of logic;

• The systems applies logical deduction to find answers for theproblem



PROLOG PROGRAMS



In a PROLOG program: A FACT represents a unit of information that is

assumed to be true;

A RULE represents a conditional assertion A VARIABLE represents an unspesified element of a

world A CONSTANT represents a spesific elements of that

world A PREDICATE represents a relatioship between

element or a property of a class of elements. A PROCEDURE is a group of clauses having the

same head predicate.



In general, PROLOG have the following syntax rule (formost PROLOG versions) :

User-defined symbols begin with a letter which may be

followed by zero or more (lower- or uppercase) letters,digits, or underscores

Variables, and only variables, begin with a capital letteror an underscore. Anonymous variables are allowed

and are represented by single underscores; each soleoccurrence of the underscore symbol represents adifferent variable (even in same clause). Clauses arespecial formulas and composed of a nmber ofelementary formulas that are called atomic formulas



l i kes( sue, husband( ann) ) ,

husband is not a constant or a variable, it is a

function symbol. A function symbols represent a way of reffering to an

element indirectly, via other elements.husband( ann) refers to an element, not by its

name, but by its relation to (represented by ) ann. Syntactically, a term never stands alone, but always within

some atomic formula .



A Term is either a variable or a constant or anexpression of the form f (T1,T2,…,Tn) where f is afunction symbol and T1, T2, …, Tn (N > 0) are terms.

The symbol f is called the functor , n the arity , and are T1, T2, …, Tn arguments of the terms.

An atomic formula is an expression of the form p(T1,

T2, …, Tn), where p is predicate symbol and T1, T2,…, Tn are terms. If n = 0 that is, no argument exist,then the parentheses are omitted



A PROLOG program is a finite set of clauses of theform :

A : - B1, …, Bn

Where n ≥ 0 and A and the Bi’s are atomic formulas. Theabove clause is read

“ A if B1 and … and Bn“.

The atomic formula A is called the head of the clauseand (B1,… Bn) is called the body of the clause. If n =0, then the clause, with the symbol “:-“ omitted, iscalled a unit clause



Example 1. The world of Ann & Sue is describedbelow

Ann likes every toy she plays with.Doll is a toy.Snoopy is a toy. Ann plays with Snoopy.Sue likes everything Ann likes



The above description is translated intoPROLOG program.

l i kes( ann, X) : - t oy( X) ,pl ays( ann, X) .

t oy( dol l ) .

t oy( snoopy) .pl ays( ann, snoopy) .l i kes( sue, Y) : - l i kes( ann, Y) .



Queries



Given the program in example 1, one may seekinformation by asking questions such as

“What does Sue like ?”

In PROLOG, the above query is written as follows :?- l i kes( sue, X) .

PROLOG, applying logical deduction to find ananswer for the query. PROLOG’s answer will be :

X = snoopy



A query has a the form ?- A1,…,An, where each Ai

is an atomic formula. In This query, (A1,…,An )

is called a goal , and each Ai is subgoal .

If n = 1, then the goal is called a simple goal .If n = 0, we have an empty goal,which is denoted

by □.



WORKING WITH AMZIPROLOG



Amzi! Listener (ALIS) a Prolog interpreter The listener directly executes source

code, which is stored in a plain text file(PRO file), or typed in directly at the ?-prompt

Amzi! Standalone Executable Runtime(ARUN)

The Amzi! runtime executes a Prolog

load module (XPL file)



Enter and edit your source code (using File/New tocreate a new file)

Consult your source code file into the listener (usingListener/Consult)

Issue Prolog queries to test your code and use thelistener debugger, as needed (using Listener/Debugon)

Respond to runtime errors by either continuing orfailing

Modify your source code to fix the problems Reconsult your source code file into the listener (using

Listener/Reconsult, which will automatically save allmodified source files/windows)

Goto step 3 until your module works



DEBUGGER

The Amzi! debugger is used to find and identifyerrors.

It works on interpreted source code. The debugger can only be invoked from, and

used within, the listener. In order to stepthrough code, you need to consult the sourcecode form of the module



INTERMIXING INTERPRETED AND COMPILED CODE

The Amzi! listener differs from other Prolog listeners because,can consult both source code files (*.PRO) and compiled object

code (*.PLM) files. This allows, to keep code underdevelopment in source form, and debugged code in compiledform.

The Listener is the heart of the Amzi! development tools. It is where you develop, test and debug your Prolog code. The textfile ENV.PRO is consulted automatically for you when thelistener starts up, which initializes various flags and options.

ENV.PRO in turn loads UTIL.PLM which contains a numberof useful predicates like member and append (see UTIL.PROfor a complete list).



Amzi! Compiler (ACMP)

Compile source files

(*.PRO) into objectfiles (*.PLM),

Compiled code runs at

least 10 times fasterthan interpreted code



Amzi! Linker (ALNK) Once an application has been debugged, the

object files (PLM) are linked into a singleexecutable load module (XPL file) that can beembedded in a C/C++, Visual Basic or otherprogram, or run as a standalone program

The linker automatically links in a copy of thestandard Amzi! library AMZILIB.PLM intoeach XPL file



PREDEF. FUNC. &

PREDICATES



PROLOG allows arithmetic terms and formulasto be written in infix order, instead of prefix

order.

Example.+( 1, *( 2, 3) ) is equivalent to 1 + 2 * 3is( X, +( Y, 1) is equivalent to X is Y + 1



?- 3 = 1 + 2noComment : 3 is not the same terms as 1 + 2

?- 3 i s 1 + 2yesComment : 3 is the value of 1 + 2.

The Predicate symbols “=“ does not represent thenormal “equality” but it represent a specialrelation in PROLOG called “unifiability”



Two Term T1 and T2 are unifiable (written T1 = T2 ) if theycan be made identical by a substitution of their

variables with appropriate terms

Ex.

?- X + 2 = 1 + Y

X = 1

Y = 2



The Predicate “ = “ unifies itstwo arguments with noarithmetic computation involved

The predicate “ is “ unifies itsfirst argument with the(computed) value of its secondargument

?- 4- 1 i s 1+2no?- 4- 1 =: = 1+2yes?- X+2 = 1+2X = 1?- X + 2 =: =1+2

no



Problem VS Solution



Translate the following sentences into a PROLOG program :Everyone who teaches a computing unit is smart.John teaches the unit MA1John’s wife teaches the unit SA1MA1 is a mathematics unitSA1 is a computing unit

From the above PROLOG program, identify the FACT,RULES, TERMs, and ATOMIC FORMULA. Also list VARIABLEs,CONSTANT, FUNCTIONS, and PREDICATEs.

Load the program to a PROLOG system and enter a query toask if ANYONE IS SMART. What is the logical meaning of theanswer



smar t ( X) : -t eaches( X, Y) , comput i ng( Y) .

t eaches( j ohn, ma1) .t eaches( wi f e( j ohn) , sa1) .mat hemat i cs( ma1) .comput i ng( sa1) .



FACTS teaches(john,ma1). teaches(wife(john),sa1). mathematics(ma1). computing(sa1).

RULE smart(X) :- teaches(X,Y),computing(Y).



ATOMIC FORMULAs smart(X), teaches(X,Y), computing(Y).

TERM Variabels : X, Y Constants : john, ma1, sa1

Function : wife(john) function symbol :wife

PREDICATEs: smart, teaches, computing



To ask if there is anyone smart ?

?- smar t ( X)X = wi f e( j ohn) .

no



Consider the following English version

Every mother likes her child if her child is goodEvery mother is woman

Ann is womanAnn’s husband is good.

Translate the above sentences into two different PROLOGprograms : one contains function symbols and the other doesnot.

Load the program to a PROLOG system and enter a query toask if there is any woman who likes someone’s husband.What is the logical meaning of the answer



Program 1

likes(mother(X),X) :- good(X).

woman(mother(X)).

woman(ann).good(husband(ann)).

Program 2

likes(X,Y) :- mother(X,Y), good(Y).woman(Y) :- mother(X,Y).

woman(ann).

good(X) :- husband(X,ann)



the program 1 is more expensive, because itsfunctions allow reference to a large number ofentities such as Ann’s husband, Ann’s mother, and

Ann’s mother-in-law, whereas in program 2, theonly entity that can be displayed is ann .

Is there any woman who likes someone’s husband ?

?- woman(X), likes(X,husband(Y)).X = mother(husband(ann)).

X = ann



?- woman(X), husband(Y,Z), likes(X,Y).no

because the system is unable to find anyone’s husband

to do this we must introduce new constant, say ann_husband,& ann_mother_in_law

husband( ann_husband, ann) .mot her ( ann_mot her _i n_l aw, ann_husband)

?- woman(X), husband(Y,Z), likes(X,Y).X = ann_mot her _i n_l aw

Y = ann_husbandZ = ann



The unification process of PROLOG assumes every function isone-to-one, that is f(x) = f(y) only if x = y. Consider thefollowing pairs of expressions and determine if they are

unifiable. If so, Find appropriate variable substitution !

a. loves(X,husband(Y)); loves(mother(Z),Z);

b. loves(X,husband(X)); loves(mother(Z),Z);

c. mother(john); mother(sue);

d. min(log(X),Y); min(U,exp(V)).

e. X + 1; Y + 2;

Bab 4 Prolog

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