Using Subroutines In Index Definitions

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It is possible to call a jBC subroutine from an index definition. The subroutine should have five parameters as follows:

Result This is to return the result of the calculation.
Filevar File variable of file for which the update is being processed.
Record Record being updated.
Key Record key of the record being updated.
Field Field, or attribute, already extracted as part of the index definition

As an example, consider the following index creation


When an update occurs the index key is calculated by taking attribute 1 and concatenating it with the output from a call to a subroutine called INDEX-DEF. The source code for this may look something like this:

001 SUBROUTINE INDEX-DEF(result , file , record , key , field )
002 IF NUM(field) THEN result = "*1" ELSE result = "*0"
003 result := record<3>

In the above example the result is created in the first parameter, the "result" variable. This is calculated by taking the string "*1" or "*0" and concatenating it with attribute 3 from the record being updated. The choice of "*1" or "*0" depends upon whether the extracted attribute, passed in the fifth parameter as variable "field" , is numeric or not. The index definition was "CALL(2,"INDEX-DEF")" so this extracted attribute will be attribute 2.

Any normal jBC code will execute in these subroutines but you should be aware of the following pitfalls.

The code should always create exactly the same result given the same record. This means you should avoid using functionality that creates a variable value, such as the RND() function, the TIME() or DATE() functions , the users port number and so on. If this occurs then there will be no way jBASE can delete a changed index value and so the index file will continually grow with invalid data even if the number of records remain constant.

These subroutines will be called implicitly from other running jBC code which to its knowledge has merely executed a DELETE or WRITE statement. You should therefore avoid any code that changes the nature of the environment such as using the default select list, turning echo on or off, turning the break key on or off. There are ways around many of these, for example you can turn the echo on and off so long as your code remembers in all cases to restore it to its original status. Similarly you can do a SELECT so long as it is to a local variable rather than the default select list.

Depending upon your application, these subroutines may be accessed by users other than the account in which the files exist. Therefore all persons who have access to OPEN and update the file must also have access to be able to CALL your subroutine. This can be done in a number of ways.

All users who want to update the file may have the environment variable JBCOBJECTLIST set to include the library where these subroutines were catologed into. For example if the subroutines have been cataloged from account greg then you can set up the JBCOBJECTLIST as follows so that we look in the users current lib directory and failing that in the lib directory for greg (this from the Korn shell)

export JBCOBJECTLIST=$HOME/lib:~greg/lib

You can CATALOG into a directory that is common to all users anyway. Such as directory is the lib directory where jBASE is installed. In this example you don't need to set up JBCOBJECTLIST. You do , however, remember to re-catalog all these subroutines when a new version of jBASE is loaded. You change the output directory for CATALOG with the JBCDEV_LIB environment variable. For example, from the Unix Korn shell you would do this: