Introduction to Prolog read, write, assert, retract. CS171: Expert Systems

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1 Introduction to Prolog read, write, assert, retract CS171: Expert Systems

2 Topics: Read and write predicates Assert and retract predicates Cuts Tracing Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 2

3 Write predicate write( ) Writes a single term to the terminal. For example: write(a), or write( How are you? ) write_ln() Writes a term to the terminal followed by a new line. tab(x) Writes an X number of spaces to the terminal. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 3

) write_ln() Writes a term to the terminal followed by a new line.

4 Read predicate read(x) Reads a term from the keyboard and instantiates variable X to the value of the read term. This term to be read has to be followed by a dot. and a white space character (such as an enter or space). For example: hello :- write( What is your name? ), read(x), write( Hello ), tab(1), write(x). Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 4

and a white space character (such as an enter or space).

5 Assert predicate assert(x) asserta(x) assertz(x) Adds a new fact or clause to the database. Term is asserted as the last fact or clause with the same key predicate. Same as assert, but adds a clause at the beginning of the database Exactly the same as assert(x) a being the first letter and z being the last letter of the alphabet should remind you where in the database you are adding a new fact or a clause. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 5

Same as assert, but adds a clause at the beginning of the database Exactly the same as assert(x) a being the first letter

6 Assert predicate For example: :- dynamic good/2. :- dynamic bad/2. assert(good(skywalker, luke)). assert(good(solo, han)). assert(bad(vader, darth)).?- listing(good). Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 6

7 Retract predicate retract(x) retractall(x) For example: retract(bad(vader, darth)). retractall(good(_, _)).?- good(x, Y). No Removes fact or clause X from the database. Removes all facts or clauses from the database for which the head unifies with X. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 7

No Removes fact or clause X from the database.

8 Cuts! is a Prolog feature called the cut. Cuts may be inserted anywhere within a clause to prevent backtracking to previous subgoals. For example: a(x) :- b(x), c(x),!, d(x), e(x). Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 8

9 Cuts What do cuts do? Suppose that clause a() from the previous slide has been invoked and the subgoals b(x) and c(x) have been satisfied. On encountering the cut: The cut will succeed and Prolog will try to satisfy subgoals d(x) and e(x). If d(x) and e(x) succeed then a(x) succeeds. If d(x) and e(x) do not succeed and backtracking returns to the cut, then the backtracking process will immediately terminate and a(x) fails. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 9

On encountering the cut: The cut will succeed and Prolog will try to satisfy subgoals d(x) and e(x).

10 Cuts For example: max(a,b,b) :- A < B. max(a,b,a).?- max(3,4,m). M = 4 ; M = 3 Using a cut: max(a,b,b) :- A < B,!. max(a,b,a).?- max(3,4,m). M = 4 ; No Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 10

11 Cuts Cuts are commonly used in the generate-and-test programming paradigm: find_just_one_solution(x) :- candidate_solution(x), test_solution(x),!. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 11

12 Cuts cut fail combination cab be used to express negation: For example: nonsibling(x, Y) :- sibling(x, Y),!, fail. nonsibling(x, Y). Fail A predicate that always returns false (i.e. it always fails, causing the whole clause to fail and Prolog to try another branch in the recursion tree. Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 12

13 Trace predicate The trace predicate prints out information about the sequence of goals in order to show where the program has reached in its execution. Example (see trace_example.pl on the course web site) Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 13

14 Trace predicate Some of the events which may happen during a trace: CALL: EXIT: REDO: FAIL: A CALL event occurs when Prolog tries to satisfy a goal An EXIT event occurs when some goal has just been satisfied A REDO event occurs when the system comes back to a goal, trying o re-satisfy it A FAIL event occurs when a goal fails Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 14

satisfied A REDO event occurs when the system comes back to a goal, trying o re-satisfy it A FAIL

15 Reference Clocksin, W.F., and Mellish C.S. Programming in Prolog. 4th edition. New York: Springer-Verlag Van Le, T. Techniques of Prolog Programming. John Wiley & Sons, Inc Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside 15

Techniques of Prolog Programming. John Wiley & Sons, Inc. 1993.

Introduction to Prolog Reading and writing. CS181: Programming Languages

Introduction to Prolog Reading and writing CS181: Programming Languages Topics: Reading and writing Prolog in action: Searching a maze Prolog in action: Searching directed graphs Vladimir Vacic, Christos