46.2.3 Retrieving Variable Values

After the successful return of PrologNextSolution, the values assigned to the variables of the query can be retrieved by specific functions of the interface. There are separate functions for retrieving the variable values in string, quoted string and integer formats.

The PrologGetLong function retrieves the integer value of a given variable within a query and assigns it to a variable. That is, the value of the given variable is converted to an integer. Returns 1 on success.

Example: The following code fragment assigns the value 2 to the variable v:

     Dim qid As Long
     Q = "member(X,[1,2,3]), X > 1"
     qid = PrologOpenQuery(Q)
     Call PrologNextSolution(qid)
     Call PrologGetLong(qid,"X",v)

The PrologGetString function retrieves the value of a given variable in a query as a string. That is, the value of the variable is written out using the write/2 Prolog predicate, and the resulting output is stored in a Visual Basic variable. Retuns 1 on success.

Example: suppose we have the following clause in a Prolog program:


The code fragment below assigns the string "Sweden-Stockholm" to the variable capital:

     Dim qid As Long
     Q = "capital_of(Expr)"
     qid = PrologOpenQuery(Q)
     If PrologNextSolution(qid) = 1 Then
       Call PrologGetString(qid,"Expr",capital)
     End if
     Call PrologCloseQuery(qid)

The PrologGetStringQuoted function is the same as PrologGetString, but the conversion uses the writeq/2 Prolog predicate. Returns 1 on success.

Example: if the function PrologGetStringQuoted is used in the code above instead of the PrologGetString function, then the value assigned to the variable capital is "'Sweden'-'Stockholm'".

The only way of transferring information from Prolog to Visual Basic is by the above three PrologGet... functions. This means that, although arbitrary terms can be passed to Visual Basic, there is no support for the transfer of compound terms such as lists or structures. We will show examples of how to overcome this limitation later in the manual (see Examples).