Question

I have the following Database which connects streets via switches:

For Example

switch(r2,w52=s).
switch(w52=s,w53=d).

How can it be determined algorithmically, not through explicit deposit in a data base.??
Any suggestions?

Answer 1

You have several options to create and consult dynamically-generated facts in Prolog.

The basic one is to use meta-predicates such as assert and retract. There predicates add and remove facts from the program. E.g., assert(switch(w52=s,w53=d)) will add a switch clause to your program. You have to declare :- dynamic switch/2. in advance. The downside of using assert is that it does not backtrack. That is, if you asserted some fact in some predicate and it backtracked, the fact is not automatically cleaned up.

Another option is to accumulate these facts inside a list, and then use member/2 or memberchk/2 to check if a given fact is in that list, instead of querying the program. E.g.:

L = [switch(w52=s,w53=d), switch(w53=d,w54=d), ...]
member(switch(w53=A, w54=B), L) % Unifies A=d, B=d

Unlike using assertions, this method will work like any other Prolog predicate.

Finally, you can run a two-phase program. During the first phase, print to a file the terms you calculated and want to serve as facts. That’s easy because Prolog supports writing complete terms, no need to format them yourself. Afterwards just consult the file. If you read the facts more often than generate them, this is the most efficient method, since Prolog will compile your file.