Prolog if/else statement with "or" condition

1
votes

I'm new to Prolog, and I'm trying to write an if/else statement with an "or" condition. So to demonstrate, I want something like:

 gothrough([H|T], B, C):-
    (  T == [] or H == 'then'  %if either the tail is an empty list or if H == "then", do the following%
    -> append(H,B,B), outputs(B,C)
    ;  append(H,B,B), gothrough(T, B, C) %else%
    ).

This implementation doesn't work however; is there an obvious way to do this that I'm not getting?

Thanks!

1
if-statementprolog

1 Answers

2
votes

In Prolog, use ";" for or and "," for and.

gothrough([H|T], B, C):-
    (  (T == [] ; H == 'then')  %if either the tail is an empty list or if H == "then", do the following%
    -> append(H,B,B), outputs(B,C)
    ;  append(H,B,B), gothrough(T, B, C) %else%
    ).

Notice that append(H, B, B) always fails when H is different of [].

You can write

 gothrough([H|T], B, C):-
    append(H,B,B), 
    (  (T == [] ; H == 'then')  %if either the tail is an empty list or if H == "then", do the following%
    -> outputs(B,C)
    ;  gothrough(T, B, C) %else%
    ).