JESS: Re: Defquery problem

From: Michael Knapik

> (defquery find-conditions

> (declare (variables ?wantedConditionCode))

> (event (conditionCodeId ?ccId&:( = ?wantedConditionCode))))

1. The = conditional element (CE) seems wrong.

It reduces to (= ?x), which is a vacuous test that always returns TRUE.

A test CE doesn't "inherit" the variable name; you have to write it

explicitly. It should be:

1.' ?ccId&:(= ?ccId ?wantedConditionCode)

(Note: The Jess in Action book, p.391, states that the math functions,

including the = function, takes argument list <number> <number>+,

where the '+' is regexp syntax for "1 or more". Actually, { -, /, ** } takes

<number>+, and are OK with 1 arg but throw an exception with 0 args,

while { +, *, =, <, >, <=, >=, <> } take <number>*, and are fine even with

0 args. (+) returns 0, (*) returns 1, and the (in)equalities with 0 or 1 arg

always return TRUE.)

2. The = CE is unnecessary. Just match on the variable directly:

(defquery find-conditions-tersely

(declare (variables ?c))

(event (conditionCodeId ?c)))

In retrospect, I don't know why your query generated nothing.

In my test, version #1 errs to the other extreme: it matched _every_

occurrence of the template, regardless of the value passed in.

Version #2 worked properly.

Eric Wang

CAD Lab, School of Mechanical Engineering, Sungkyunkwan University

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