Fun with varying length fields

So here’s a really simple little program that puts the first five characters of a ten character field into a five character field. So why doesn’t it work?

      **Free

        // Fun with variable length fields

        ctl-opt dftactgrp(*no) actgrp(*new)
            main(Main);

        dcl-proc Main;
            dcl-pi *n end-pi;

            dcl-s LongString varchar(10) inz('123');
            dcl-s ShortString char(5) inz;
                                                             
            shortstring = %subst(longstring: 1: 5);                 
            dsply shortstring;                                      
                                                             
            return;                                                 
        end-proc;

If you compile and run this program, it will return an RNX0100 error: “Length or start position is out of range for the string operation”.

The reason, of course, is that LongString is a varying length field. The ten character length in the field definition defines the maximum size but, because the field is initialised with a three character value, the length of the field when the program starts is only three characters. And using the %subst bif to find the first five characters of a three character field will, of course, return an error.

The above program is so simple that the problem probably leapt out at you but when you are dealing with a bunch of externally defined fields, gotchas like this are less immediately apparent. So take this as a reminder to be wary of documentation and to familiarise yourself with the DSPFFD command.