assert and retract

1
assert and retract

• Modifying Programs Dynamically
• Prologs Execution Tree

2
Built-ins to Modify Prolog Programs

• The following built-ins allow modification of
• the loaded program as it is actually running.
• assert(C) - adds a clause C to program
• asserta(C) - C is added to front
• assertz(C) assert(C) - C is added to end
• They always succeed.
• retract(C) - deletes a clause that matches C
• It fails if there is nothing to delete

3
Examples of using assert/retract

• ?-nice.
• ?-disgusting.
• ?-retract(fog).
• ?-disgusting.
• ?-assert(sunshine).
• ?-funny.
• ?-retract(raining).
• ?-nice.

• nice- sunshine,
• \ raining.
• \ means not
• funny- sunshine,
• raining.
• disgusting- raining, fog.
• raining.
• fog.

4
Examples of using assert/retract

• Fibonacci numbers 1,1,2,3,5,8,13,21,34,
• fib(N,F) - F is the N'th Fibonacci number.  
• fib(1,1).
• fib(2,1).
• fib(N,F) - N gt 2, N1 is N-1, N2 is N-2,
  fib(N1,F1), fib(N2,F2), F is F1 F2.

5
Execution Tree to Compute fib(6,F)

• fib(6,F)
• fib(5, F1) fib(4,F2) F is F1F2

6
To improve the above program, we can use
assert to save intermediate results

• - dynamic myfib/2.
• myfib(1,1).
• myfib(2,1).
• myfib(N,F) - N gt 2, N1 is N-1, N2 is N-2,
  myfib(N1,F1), myfib(N2,F2),
• F is F1 F2,
• remember n-th Fibonacci number asserta(
  myfib(N,F) ).

7
assert/retract plays two roles

• Adding new information to the database
• (ref. previous examples)
• Communicating between or-branches
• An example ?- b(X),b(Y).
• b(0).
• b(1).

8
the following program collects a list of boy names

• make_name_list(NameList)-
• initialise the name list as an empty list
• assert(tmp()),
• boy_names(X, _, _),
• retract(tmp(CurrentList)),
• assert(tmp(XCurrentList)),
• fail.
• make_name_list(NameList)- retract(tmp(NameList)).

9
program collects all solutions of cango(X, Y,
Path)

• cango_all(X, Y, _AllRoutes)-
• initialise
• assert(tmp()),
• cango(X, Y, Path),
• retract(tmp(CurrentList)),
• assert(tmp((X,YPath)CurrentList)),
• fail.
• cango_all(_, _, AllRoutes)- retract(tmp(AllRoutes
  )).

10
How to use findall

• Prolog has provided a built-in predicate
  findall(Vars, Goal, S) which collects all
  solutions of Goal into S.
• Here is an example
• test(X,Y, AllRoutes)-
• findall( (P,L), cango(X,Y,P,L), AllRoutes).
• This returns
• AllRoutes (path1,line1), (path2,line2),
• where pathi and linei are lists.