PROLOG: Database Manipulation In Prolog

1. Database manipulation in Prolog

3. bagof/3

4. setof/3Examples

6. retract

7. asserta

9. Suppose we then made four more assert commands: assert(happy(vincent)). yes assert(happy(marcellus)). yes assert(happy(butch)). yes assert(happy(vincent)).  yes Suppose we then ask for a listing: listing.  happy(mia). happy(vincent). happy(marcellus). happy(butch). happy(vincent). yes

10. assert So 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. retract To remove all of our assertions we can use a variable: retract(happy(X)). X = mia ; X = butch ; X = vincent ; No listing.  Yes (reveals that the database is now empty)

12. assertz and asserta If 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 Solutions There 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).

15. bagof

16. SetofThese predicates collect all the solutions to a query and put them in a list,

17. findall/3 findall(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 = _7489 Z = [charlotte,caroline,laura,rose]

19. Prolog will respondX = _7616 Z = [fromMartha(charlotte),fromMartha(caroline), fromMartha(laura),fromMartha(rose)]

20. bagof/3 suppose we pose the query findall(Child,descend(Mother,Child),List). We get the response Child = _6947 Mother = _6951 List = [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 query bagof(Child,descend(Mother,Child),List). we get the response Child = _7736 Mother = caroline List = [laura,rose] ; Child = _7736 Mother = charlotte List = [caroline,laura,rose] ; Child = _7736 Mother = laura List = [rose] ; Child = _7736 Mother = martha List = [charlotte,caroline,laura,rose] ; no

22. bagof/3 If 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 = _7870 Mother = _7874 List = [charlotte,caroline,laura,rose,caroline,laura,rose, laura,rose,rose]

23. setof/3 In setof/3 predicate, the lists contained are ordered and contain no redundancies. Ex:suppose we have the following database age(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 = _8443 Y = _8448 Out = [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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