Commodore 1581 - Chapter 5: Relative Data Files; Files, Records, and Fields; File Limits for Relative Files

records are gone too, replaced by subdivisions within the records,
called fields. Each field is large enough to hold one piece of informa
tion about one record in the file. Thus, within each file there are many
records, and within each record there are typically many fields.
A relative file takes care of organizing the records for you, num
bering them from 1 to the highest record number, by ones, but the
fields are up to you to organize. Each record will be of the same size,
but the 1581 won't insist that they all be divided the same way. On the
other hand, they normally will be subdivided the same way, and if it
can be known in advance exactly where each field starts within each
record, there are even fast ways to access a desired field within a
record without reading through the other fields. As all of this implies,
access speed is a primary reason for putting information into a relative
disk file. Some well-written relative file programs are able to find and
read the record of one desired person out of a thousand in under 15
seconds, a feat no sequential file program could match.
FILE LIMITS
With relative files, you don't have to worry about exactly where
on the diskette's surface a given record will be stored, or whether it
will fit properly within the current disk sector, or need to be extended
onto the next available sector. DOS takes care of all that for you. All
you need to do is specify how long each record is, in bytes, and how
many records you will need. DOS will do the rest, and organize things
in such a way that it can quickly find any record in the file, as soon as it
is given the record number (ordinal position within the file).
The only limit that will concern you is that each record must be
the same size, and the record length you choose must be between 2
and 254 characters. Naturally the entire file also has to fit on your
diskette, along with any other existing file(s).
CREATING A RELATIVE FILE
When a relative file is to be used for the first time, its Open
statement will create the file; after that, the Open statement is used to
reopen the file for both reading and writing.
FORMAT STATEMENT TO OPEN A RELATIVE FILE:
BASIC 7.0: DOPEN # file #, "file name", L record length
[JDdrive #] [,Udevice #]
BASIC 2.0: OPEN file #, device #, channel #, "drive #: file name,
L," + CHR$ (record length)
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