Chapter 6

Control Backtracking

Chapter 6 2344-302 LP and Prolog

The Cut !

Cut is the built-in predicate that instructs the interpreter not to backtrack beyond the point at which it occurs.The cut is used to reduce the size of the search space of a query.Cut may effect :

on a compound query oron the set of clauses.

Chapter 6 3344-302 LP and Prolog

Cut

Compound query Get a value of X from “a”Get a value of Y from “b”Get a value of X,Y,Z from “c”

Goal : a(X), b(Y), ! , c(X,Y,Z).

It will not go beyond ! command

Chapter 6 4344-302 LP and Prolog

predicates a(integer) b(integer) c(integer,integer,integer)

run1 /* when success then stop */ run2 /* find all solution , backtrack all "b" value first before

backtrack "a" value */ run3 /* not go further than ! */ run4 /* fail and cut work together */ run5 /* fail and cut work together, not go to "a" value */

clauses a(3). a(7). a(9). b(10). b(200). c(S,T,U) :- U = S + T. run1 :- a(X), b(Y), c(X,Y,Z), write("a = ", X," b = ", Y, " c = ",Z), nl. run2 :- a(X), b(Y), c(X,Y,Z), write("a = ", X," b = ", Y, " c = ",Z), nl, fail. run3 :- a(X), b(Y), !, c(X,Y,Z), write("a = ", X," b = ", Y, " c = ",Z), nl. run4 :- a(X), b(Y), !, c(X,Y,Z), write("a = ", X," b = ", Y, " c = ",Z), nl, fail.

run5 :- a(X),!, b(Y), c(X,Y,Z), write("a = ", X," b = ", Y, " c = ", Z), nl, fail.

LP24.pro

Cut

Chapter 6 5344-302 LP and Prolog

Factorial Ex07EX03.pro

/* Recursive program to compute factorials. */

predicates factorial(integer, real)clauses

factorial(1, 1) :- !. factorial(X, FactX)

:- Y = X-1, factorial(Y, FactY),

FactX = X*FactY.

Chapter 6 6344-302 LP and Prolog

Factorial (2)

/* Recursive program to compute factorials. */

predicates factorial(integer, integer)clauses

factorial(1, 1) :- !. factorial(FactX, X )

:- Y = X-1, factorial( FactY, Y),

FactX = X*FactY.

Chapter 6 7344-302 LP and Prolog

Evaluation of a query

Recursive cycle of unification (pattern matching)

Subgoal evaluationEvaluate the body of a clauseIf unification fail, go on to the next clause

Ex: uncle(john,daniel) :- brother(john,bill),

father(bill, daniel)

Chapter 6 8344-302 LP and Prolog

Unification

1. A variable unifies with a constant? X = john

2. A variable unifies with a variable? X = Y

3. _ unifies with anything? likes(_,What)

Chapter 6 9344-302 LP and Prolog

Unification

4. A constant unifies with a constant, if they are identical? likes(john, dog)

yes5. A structure unifies with a structure if

the structure names are the same and if the arguments can be unified.

? father(john) = father(X)X = john

Chapter 6 10344-302 LP and Prolog

Failure and backtracking

If the active query is part of compound query (a list of subgoal) and is not the first subgoal in the compound query

then the interpreter backtracks to reconsider the previous subgoal in the

compound query.If the active query is the first subgoal in the compound query, then when it is fail, the compound query fail as well.

Chapter 6 11344-302 LP and Prolog

predicates uncle(symbol,symbol) brother(symbol,symbol) father(symbol,symbol)clauses brother(daniel, kenneth). brother(daniel, jim).

brother(john,jim). father(bill, daniel).father(kenneth, bill ).

uncle(U,N) :- brother(U,B), father(B,N).

LP Lec20.pro

Chapter 6 12344-302 LP and Prolog

predicates register(symbol) age(symbol,integer) male(symbol)clauses

male(brain). male(mike). male(steve). age(brain, 18). age(mile, 17). age(steve, 18). register(X) :- male(X), age(X,Y), Y = 18.

LP Lec20.pro

Chapter 6 13344-302 LP and Prolog

Recursive procedures

1. A nonrecursive clause defining the base case of the procedure, that is where the recursion stops.2. A recursive rule. In the body of this rule,

the first subgoals generate new argument values. Then follows a recursive subgoal utilizing the new argument values.

Chapter 6 14344-302 LP and Prolog

case 1ancestor(A,C) :- parent(A,C). /*base case */

ancestor(A,C) :- parent(A,B),ancestor(B,C).

Recursive procedures

case 2ancestor(A,C) :- parent(A,C). /*base case */

ancestor(A,C) :- ancestor(B,C), parent(A,B).

OK

OK

Chapter 6 15344-302 LP and Prolog

Case 1 : lp23c1.pro : Top down

predicates ancestor1(symbol,symbol) parent(symbol,symbol)clauses parent(ellen, john). parent(ellen, lisa). parent(lisa, sherry). parent(john, tom). parent(tom, eric). parent(eric, mark). /*......... case 1.............. */ ancestor1(A,C) :- parent(A,C). /* base case */ ancestor1(A,C) :- parent(A,B), ancestor1(B,C). /* recursive

clause */

Q1 ?ancestor1(ellen,tom)

Q2 ?ancestor1(ellen,mark)

Q3 ?ancestor1(ellen,sherry)

Yes

Yes

Yes

Chapter 6 16344-302 LP and Prolog

Family Tree

ellen

john Lisa

tom sherry

eric

mark

L1

L2

L3

L4

L5

TOP

Bottom

case 1

case 2

Chapter 6 17344-302 LP and Prolog

Case 2 : lp23c2.pro : Bottom up

predicates ancestor2(symbol,symbol) parent(symbol,symbol)clauses parent(ellen, john). parent(ellen, lisa). parent(lisa, sherry). parent(john, tom). parent(tom, eric).

parent(eric, mark). /*......... case 2 ..............*/ ancestor2(A,C) :- parent(A,C). /* base case */ ancestor2(A,C) :- ancestor2(B,C), parent(A,B). /* recursive

clause */

Q1 ?ancestor2(ellen,tom)

Q2 ?ancestor2(ellen,mark)

Q3 ?ancestor2(ellen,sherry)

Yes

Yes

Yes

Chapter 6 18344-302 LP and Prolog

Both people like the same thingpredicates both_like(symbol,symbol,symbol) like(symbol,symbol) same_thingclauses both_like(First,Second, Thing) :- like(First,Thing), like(Second,Thing), First <> Second. same_thing :- both_like(First,Second,Thing), write(First, " and ",Second," like ",Thing, "\n \n"). like(kosin,water). like(dum,wine). like(dum,salad). like(nipa,salad). like(nipa,ice_cream). like(nipa,wine).

Chapter 6 19344-302 LP and Prolog

Count down case 1

predicates countdown1. numb(integer).clauses numb(5).

numb(4).numb(3).numb(2).numb(1).numb(0).

countdown1 :- numb(X), write(X," "), fail.

Chapter 6 20344-302 LP and Prolog

Count down case 2 (p.44 thai bk)

predicates countdown(integer). clauses countdown(0) :- write("0 "). countdown(N) :- write(N," "), M = N - 1,

countdown(M).

Chapter 6 21344-302 LP and Prolog

Count up case 1 (p.48 thai bk)

predicates countup1. numb(integer).clauses numb(0).

numb(1).numb(2).numb(3).numb(4).numb(5).

countup1 :- numb(X), write(X," "), fail.

Chapter 6 22344-302 LP and Prolog

Count up case 2 (p.49 thai bk)

predicates countup2. numb(integer).clauses

countup2 :- numb(X), write(X," "), fail.

numb(0). numb(A) :- numb(B), A = B + 1.

Use Fail

Loop until …………infinity

Chapter 6 23344-302 LP and Prolog

Count up case 3 (p.49 thai bk)

domains X = integer predicates countup3(integer). numb(integer).clauses

countup3(N) :- numb(X), write(X," "), X = N.

numb(0). numb(A) :- numb(B), A = B + 1.

Do until X = N

Loop until …………N

LAB : Recursive clause

/*......... แบบที่�� 1.............. */ako1 (A,C) :- isa(A,C). /* base case */

ako1 (A,C) :- isa(A,B), ako1 (B,C). /* recursive clause */

/*......... แบบที่�� 2 ..............*/ ako2 (A,C) :- isa(A,C). /* base case */ ako2 (A,C) :- ako2 (B,C), isa(A,B). /*

recursive clause */

จงเขี�ยนโปรแกรมภาษาโปรล็�อกเพื่��อพื่�สู�จน�ว่�าประโยค Recursive ในแบบที่�� 1 แล็ะ แบบที่�� 2 ที่��ก#าหนดให&ว่�าสูามารถใช้&พื่�สู�จน�การที่#างาน ako ได&เหม�อนหร�อแตกต�างก+นอย�างไรให&น+กศึ-กษายกต+ว่อย�างขี&อเที่�จจร�งขี-.นเอง พื่ร&อมอธิ�บายผล็การที่#างานที่��ได&จากการร+นโปรแกรมโดยล็ะเอ�ยด

LAB :count up / count down

จงเขี�ยนโปรแกรมภาษาโปรล็�อก 1) เพื่��อพื่�มพื่�เล็ขีค��เพื่��มขี-.นจาก จาก 0 ถ-ง 30

2) พื่�มพื่�เล็ขีค��ล็ดล็งจาก 29 ถ-ง 1

The end