PROLOG: Database Manipulation In Prolog
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Prolog has four database manipulation commands: assert, retract, asserta, and assertz. assert adds facts and rules to the database, retract removes them, and asserta and assertz control where asserted material is placed. There are three predicates for collecting all solutions to a query: findall, bagof, and setof. findall produces a list of all objects satisfying the goal. bagof produces a separate list for each instantiation of variables in the goal. setof orders solutions and removes redundancies.
![findall/3findall(Object,Goal,List).produces a list List of all the objects Object that satisfy the goal Goal.Suppose we're working with the previous database (that is, with the information about child and the definition of descend). Then if we pose the query findall(X,descend(martha,X),Z).we are asking for a list Z containing all the values of X that satisfy descend(martha,X).Prolog will respond as:X = _7489Z = [charlotte,caroline,laura,rose]](https://image.slidesharecdn.com/databasemanipulationinprolog-100328214029-phpapp01/85/PROLOG-Database-Manipulation-In-Prolog-17-320.jpg)
PROLOG: Database Manipulation In Prolog
- 1. Database manipulation in Prolog
- 3. bagof/3
- 5. Database ManipulationProlog has four database manipulation commands: assert
- 6. retract
- 7. asserta
- 8. assertzassertConsider an empty database. So if we give the command:listing.we simply get a yes; the listing (of course) is empty.Suppose we now give this command:assert(happy(mia)).It succeeds (assert commands always succeed). If we now give the command:listing.we get the listing:happy(mia).That is, the database is no longer empty: it now contains the fact we asserted.
- 9. Suppose we then made four more assert commands:assert(happy(vincent)).yesassert(happy(marcellus)).yesassert(happy(butch)).yesassert(happy(vincent)). yesSuppose we then ask for a listing:listing. happy(mia). happy(vincent). happy(marcellus). happy(butch). happy(vincent). yes
- 10. assertSo far, we have only asserted facts into the database, but we can assert new rules as well.Suppose we want to assert the rule that everyone who is happy is naive. That is, suppose we want to assert that:naive(X) :- happy(X).We can do this as follows:assert( (naive(X) :- happy(X)) ).
- 11. retractTo remove all of our assertions we can use a variable:retract(happy(X)).X = mia ;X = butch ;X = vincent ;Nolisting. Yes (reveals that the database is now empty)
- 12. assertz and assertaIf we want more control over where the asserted material is placed, there are two variants of assert, namely:1. assertz. Places asserted material at the end of the database.2. asserta. Places asserted material at the beginning of the database.For example, suppose we start with an empty database, and then we give the following command:assert( p(b) ), assertz( p(c) ), asserta( p(a) ).Then a listing reveals that we now have the following database:p(a).p(b).p(c).yes
- 13. Collecting SolutionsThere may be many solutions to a query. For example, suppose we are working with the database:child(martha,charlotte).child(charlotte,caroline).child(caroline,laura).child(laura,rose).descend(X,Y) :- child(X,Y).descend(X,Y) :- child(X,Z),descend(Z,Y).Then if we pose the query descend(martha,X).There are four solutions (namely X=charlotte, X=caroline, X=laura, and X=rose).
- 14. Collecting SolutionsProlog has three built-in predicates that do this: findall
- 15. bagof
- 16. SetofThese predicates collect all the solutions to a query and put them in a list,
- 17. findall/3findall(Object,Goal,List).produces a list List of all the objects Object that satisfy the goal Goal.Suppose we're working with the previous database (that is, with the information about child and the definition of descend). Then if we pose the query findall(X,descend(martha,X),Z).we are asking for a list Z containing all the values of X that satisfy descend(martha,X).Prolog will respond as:X = _7489Z = [charlotte,caroline,laura,rose]
- 18. Suppose we want to build a new predicate from Martha/1 that is true only of descendants of Martha. We could do this with the query:findall(fromMartha(X),descend(martha,X),Z).That is, we are asking for a list Z containing all the values of fromMartha(X) that satisfy the goal descend(martha,X).
- 19. Prolog will respondX = _7616Z = [fromMartha(charlotte),fromMartha(caroline),fromMartha(laura),fromMartha(rose)]
- 20. bagof/3suppose we pose the queryfindall(Child,descend(Mother,Child),List).We get the responseChild = _6947Mother = _6951List = [charlotte,caroline,laura,rose,caroline,laura,rose,laura,rose,rose]sometimes it would be useful if we had a separate list for each of the different instantiations of Mother.
- 21. bagof lets us do. If we pose the querybagof(Child,descend(Mother,Child),List). we get the responseChild = _7736Mother = carolineList = [laura,rose] ;Child = _7736Mother = charlotteList = [caroline,laura,rose] ;Child = _7736Mother = lauraList = [rose] ;Child = _7736Mother = marthaList = [charlotte,caroline,laura,rose] ;no
- 22. bagof/3If we pose the query bagof(Child,Mother ^descend(Mother,Child),List).This says: give me a list of all the values of Child such that descend(Mother,Child), and put the result in a list.So posing this query yields:Child = _7870Mother = _7874List = [charlotte,caroline,laura,rose,caroline,laura,rose,laura,rose,rose]
- 23. setof/3In setof/3 predicate, the lists contained are ordered and contain no redundancies.Ex:suppose we have the following databaseage(harry,13).age(draco,14).age(ron,13).age(hermione,13).age(dumbledore,60).age(hagrid,30).
- 24. Now suppose we want a list of everyone whose age is recorded in the database. We can do this with the query:findall(X,age(X,Y),Out).X = _8443Y = _8448Out = [harry,draco,ron,hermione,dumbledore,hagrid]If we want the list to be ordered, we can achive this by the following query:setof(X,Y ^ age(X,Y),Out).X = _8711 Y = _8715 Out = [draco,dumbledore,hagrid,harry,hermione,ron] here, the list is alphabatically ordered.
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