Documentation: Programming guide

Making queries

SPIRIT mainly provides two ways of making queries, both of which are described in this chapter.

Getting a Variable's Probabilities

The simplest way of querying the actual knowlegde is to compute the actual probability of a certain variable value. This can be done using the KBase.valueProb method. The following sequence computes these values for variable 0:

int arity = kb.getVarArity(0);
double[] p = new double[arity];
double rest = 1;
for(int i = 0; i < arity - 1; i++) {
		 p[i] = kb.valueProb(0,i);
		 rest -= p[i];
p[arity - 1] = rest;   »The last value is 1 - sum of the others«

Of similar interest is the expected value of a variable. This is defined as the weightened sum of the values' probabilities and can be computed using KBase.expectedValue.

Getting a Rule's Probabilities

The actual conditional probability of a rule or fact can be computed with the KBase.ruleProb method. After a successful iteration process, this value should be the rule's prescribed probability plus a tolerance given as the threshold for the iteration. For non-facts it is also possible to compute the actual premise probability with the method KBase.rulePremProb.

Assigning Evidence

So far about the ways to get the information the user might be interested in. Assigning evidence to a variable is a very simple way of querying a KBase. SPIRIT provides two methods for that purpose. First, call KBase.assignEvidence to assign evidence to certain values. After measuring the impact on the probabilities of other variables, call KBase.assignEvidenceReset to return to the original distribution. Remember that many methods are unavailable when evidence is assigned.

Complex Queries

Making complex queries means to learn additional rules (the premises) into the KBase and compute the probabilities of other rules, that were not learned before (the conclusions). Here is a general technique to make such complex queries:

  1. Add the set of premise rules to the KBase.
  2. To be able to learn those into the KBase, be sure that each of them is active. Otherwise rebuild the structure and reinit it.
  3. Then add the set of conclusions to the KBase.
  4. Then make all rules but the premises activable or passive, for that they would not be used in the following iteration process.
  5. Make a full iteration on the premises.
  6. After a successful ending, query the actual probabilities of the conclusions (they don't need to be active or activable).
  7. Then you could remove the newly added rules again and restore the old knowlegde state with a reinit. Make sure that the original rules are active.