Prolog Query: Return value based on Facts

Question

I am new to Prolog as a whole and i am learning prolog query.

I am looking at this tutorial. http://www.cs.trincoll.edu/~ram/cpsc352/notes/prolog/factsrules.html and trying to play around with my own database.

Person("Mike", "123456").   //Person(Name,ID)
Info("CA", "123456", "17"). //Info(State, ID, Age)

What I am trying to do:

I am trying to return someone's age by their name and State. by doing the following query, this should return 17.

Age_Of(Name, State, N) :- Info(State, Person(Name, X), N).

i tried the above code and it returns "No". I believe that it is still evaluate as boolean. However, I want the function to return the actual age.

(to your new, deleted Q) is_older would more truly be named twice_older. — Will Ness

damianodamiano damianodamiano · Accepted Answer · 2018-08-26T20:40:44

First of all, let's point some facts:

You should write your database of facts without using a capital letter as first letter. So Person("Mike", "123456"). should be person("Mike", "123456"). and Info("CA", "123456", "17"). should be info("CA", "123456", "17").
In prolog there are not functions but only predicates and should be defined aswell using non capital letter, so Age_Of(Name, State, N) should be age_of(Name, State, N).
Even if you remove all first capital letter, you get Singleton variables: [X] while you execute the code. This mean that you dhave defined a variable, in this case X, but not used anywhere. In this case, you should replace every singleton variable with _ or add _ before the variable (_X in this case).

To solve your problem, this is the solution:

person("Mike", "123456").   % person(Name,ID)
info("CA", "123456", "17"). % info(State, ID, Age)


age_of(Name, State, Age):-
   person(Name,ID),
   info(State, ID, Age).

So first you have to find the ID of a person with a given name and then find the corresponding age. Query:

?- age_of(Name,State,Age).
Age = "17",
Name = "Mike",
State = "CA"

This query is quite generic, you can be more specific (for instance, specifing a name). Hope it helps.

Prolog Query: Return value based on Facts

1 Answers