Prolog Mutual recursion

Question

Ok I'm writing some code to go through a check each value by mutual recursion in Prolog. This is my code so far:

semestersok(SP) :-
    [Prior|Tail] = SP,
    sem1ok(SP).



%% sem1ok(SP) :- checks semester 1 of SP is ok
sem1ok(SP) :-
    [Sem1|Tail] = SP,
    sem2ok(Tail).

%% sem2ok(SP) :-
sem2ok(SP) :-
    [Sem2|Tail] = SP,
    sem1ok(Tail).

I haven't put any code in yet to do with checking (There's two relations as it has to check alternating values), I'm having a problem with the code cycling through until it has an empty list, then it fails and comes back with false (no). Since This code isn't manipulating any code I believe it should come back true as it stands right now. Why isn't it?

Tudor Berariu · Accepted Answer · 2014-06-19T08:54:42.997

2

You need some rules for the empty list. Add this:

sem1ok([]).
sem2ok([]).

Also, the code might be more intuitive if you write it like this (as the distinction between the rule matching the empty list and the one matching the non-empty list is more clear):

% Rules for sem1ok/1
sem1ok([]).
sem1ok([Sem1|Tail]):-
    ok(Sem1), % Something involving Sem1
    sem2ok(Tail).

% Rules for sem2ok/1
sem2ok([]).
sem2ok([Sem2|Tail]):-
    ok(Sem2), % Something involving Sem2
    sem1ok(Tail).

edited Jun 19 '14 at 08:54

answered Jun 19 '14 at 08:46

Tudor Berariu

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Ok, sorry to sound dumb but where abouts do i add that? before all of this code? – Aaron Troeger Jun 19 '14 at 08:49
1

You can write any number of rules for a predicate and usually all of them are packed together. So, group the rules for `sem1ok/1` and `sem2ok/1` separately. – Tudor Berariu Jun 19 '14 at 08:53
Awesome thank you, its working now :) it's just asking for an action after it executes now which is weird, but its coming back true – Aaron Troeger Jun 19 '14 at 08:55
No wait, i found out why its doing that. All good, thank you very much! – Aaron Troeger Jun 19 '14 at 09:05

Prolog Mutual recursion

1 Answers1