Question

I’m just picking up Prolog now, so I’m unfamiliar with the normal way of doing most things.

Essentially I have a rule which gives a value from an input:

ScoreFromInput(Input, Score) :- ...

And I have a list of inputs, which are just numbers. I’m having trouble figuring out how to find the input which yields the maximum score.

This is what I have right now, but I think it recurses infinitely:

bestInput(BestInput) :-
    %#Bind the list of valid inputs
    legalInputs(ValidInputs),
    %# -1000 is a dummy score, it should get replaced on the first call to bestInputHelper
    bestInputHelper(ValidInputs,-1000,BestInput).

%#I think this rule should work if the first input in the list is not the best one
bestInputHelper([Input|RestOfInputs],BestScore,BestInput):-
    bestInputHelper(RestOfInputs,RestBestScore,BestInput),
    ScoreFromInput(Input,BestScore),
    RestBestScore > BestScore.

%#And this one if it is the best input
bestInputHelper([Input|RestOfInputs],BestScore,Input):-
    bestInputHelper(RestOfInputs,RestBestScore,_RestBestInput),
    ScoreFromInput(Input,BestScore),
    RestBestScore =< BestScore.

This is what I have so far, but I imagine there is a much more straightforward way of doing it. Any help is appreciated! Thanks!

Answer 1

A simple way to state it is that an input is best if there is no better input:

best_input(Best) :-
    legal_inputs(Inputs),
    member(Best, Inputs),
    input_score(Best, Score),
    \+ ( member(Better, Inputs), input_score(Better, S), S > Score).

To see what is wrong with your own code, try for example SWI-Prolog’s graphical tracer:

?- gtrace, best_input(Best).

And please_use_readable_names inSteadOfUnreadableOnes.