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logicanswer-set-programmingclingogringo

Fallback to default if an atom doesn't exist

Beginner to ASP here :) I need to define a default value for an atom when it isn't provided in the input.

% creating rooms
cost(create(room(R)), W) :- room(R), roomDomainNew(R), roomCost(W).

Here, if roomCost(W) is not defined, I want to fallback to a roomCost(0) zero cost. How do I do this?

Defining a roomCost(0) when the input has a roomCost of its own, leads to both co-existing.

roomCost(0).
roomCost(5).

In other languages, I could do something like:

finalRoomCost = 0
if roomCost:
    finalRoomCost = roomCost

and then I could use finalRoomCost. Is that possible here?

Solution

  • Related answer here, which says such mutability is not possible.

    In ASP truth values do not change over time, either atoms are true or false for a specific answer set, they can not be overwritten.

    So here's the solution I went with, using a #max directive:

    roomCost(0).
finalroomCost(W) :- roomCost(0), #max {X, 1 : roomCost(X)} = W.