Prolog: How to Verify User Input? - input

Question

I'm new to Prolog, and trying to implement an example from a textbook. I have been able to get the example working (through definite help from previous Stack Overflow posts!) finally, but now it is asking me to verify the user input: "Modify the given decision tree program, so that when the user responds to a question with an illegal answer, the system will ask him/her to reenter an answer which is among the specified choices."
In the below program, for marital status, the specified only options considered in the decision tree are "single" or "married." If you enter anything else, the goal is to get you to reenter your decision.
This is not working for me. My code is below:
:-dynamic income/2.
:-dynamic marital_status/2.
:-dynamic mortgage/2.
:-dynamic age/2.
marital_status(joe,married).
income(joe,60000).
mortgage(joe,20000).
age(joe,45).
main(X,Z):-var(X), write('what is your name?'),read(X), invest(X,Z),!.
main(X,Z):-invest(X,Z),!.
ask_marital_status(X,Y):-marital_status(X,Y).
ask_marital_status(X,Y):-not(marital_status(X,Y)), write('what is your marital status: married or single?'), read(Y), nl, asserta(marital_status(X,Y)).
ask_marital_status(X,Y):-Y \=married, Y \=single, write('what is your marital status: married or single?'), read(Y), nl, asserta(marital_status(X,Y)).
%ask_marital_status(X,Y):-Y \= married; Y \= single, ask_marital_status(X,Y).
ask_income(X,Y):-income(X,Y).
ask_income(X,Y):-not(income(X,Y)),write('what is your annual income?'), nl, read(Y), asserta(income(X,Y)).
ask_mortgage(X,Z):-mortgage(X,Z).
ask_mortgage(X,Z):-not(mortgage(X,Z)), write('what is your mortgage?'), read(Z), nl, asserta(mortgage(X,Z)).
ask_age(X,A):-age(X,A).
ask_age(X,A):-not(age(X,A)), write('what is your age?'), read(A), nl, asserta(age(X,A)).
moderate_risk(X):-ask_marital_status(X,Y), Y=married, ask_income(X,I), I=<50000, ask_mortgage(X,Z), Z=<50000,!.
moderate_risk(X):-ask_marital_status(X,M), M=married, ask_income(X,I), I=<50000,!.
moderate_risk(X):-ask_marital_status(X,M), M=single, ask_income(X,I), I=<35000,!.
stable_risk(X):-ask_marital_status(X,M), M=married, ask_income(X,I), I=<50000, ask_mortgage(X,Z), Z>50000,!.
stable_risk(X):-ask_marital_status(X,M), M=single, ask_income(X,I), I>35000, ask_age(X,A), A>50, !.
high_risk(X):-ask_marital_status(X,M), M=single, ask_income(X,I), I>35000, ask_age(X,A), A=<50, !.
invest(X,oil):-stable_risk(X),!.
invest(X,telecommunications):-moderate_risk(X),!.
invest(X,computers):-high_risk(X),!.
The third "ask_martial_status" is my current attempt to get the user to reenter their decision, but it is not working. I have tried it with Prolog's AND operator(,) and their or operator (;) - neither make a difference (for what I place in between Y\= married and Y\=single). When I enter erroneous input, I just get "false" returned. Below is an example:
?- main(X,Z).
what is your name?logan.
what is your marital status: married or single?|: widowed.
false.
The commented out line (with the %) was a previous attempt to get the program working that also failed. I'm surprised I wasn't able to find a quick YouTube video / article to read when I Google'd this issue. Could anyone help me out here?

Answer 1

The traditional way of reading and validating user input is to use the following template:
ask(Data) :-
repeat,
write(Prompt),
read(Data),
valid(Data),
!.
Applying this template to your case, we can write:
ask_marital_status(Status) :-
repeat,
write('What is your marital status (married or single)?'),
read(Status)
( Status == married
; Status == single
),
!.
As a general rule, is best practice to separate asking the user input from processing the input (asserting a fact in your case).
High-level solutions would use message printing and question asking mechanisms for user interaction (see e.g. https://logtalk.org/2019/11/14/abstracting-user-interaction.html). But that's a more advanced topic.

Answer 2

You can solve your problem in this way (snippet of the involved prefocate):
ask_marital_status(X,Y):-
\+ marital_status(X,Y),
write('what is your marital status: married or single?'),
read(Y1),
( (Y1 == single ; Y1 == married) ->
Y = Y1,
asserta(marital_status(X,Y));
ask_marital_status(X,Y)
).
You can check if the value you read (Y1) is single or married with (Y1 = single ; Y1 = married) (; means or). Then, if so (->) you proceed with the rest of the predicate. Otherwise, (after the ; next to asserta/1) you call recursively marital_status/2.
EDIT: thanks to Paulo Moura's comment, =/2 (unification) must be replaced with ==/2 (equality), in order to make the code behave as expected.