File Limits; Creating A Relative File; Using Relative Files: Record - Commodore 1541 User Manual

desired person out of a thousand in under 15 seconds , a feat no sequential file
could match . progralli
FILE LIMITS
. One of the nicest aspects of relative files is that all this is done for you without
0
havmg to worry at .all
~bout
exactly where on the diskette's surface a given record wiii
Ur
stored, or whether 1t will fit properly within the current disk sector or need to be ext d
he
onto. the next available sector. DOS takes care of all that for you'. All you need to
e~o ~d
specify how long
eac~
reco.rd is: in bytes, and how many records you will need . DOS
1
,/
do the rest'.
a~d ~rgan~ze
thmgs m such a way that it can quickly find any record in the
fii
11
as soon as It is ~1~en Its re~ord number
(ordinal position within the file) . e,
The only hm1t that will concern you, is that each record must be the same siz
the record length you choose must be between 2 and 254 characters Naturally theee, and
file I
h fi · . · nt1re
a so as to t on your diskette too, which means that the more records you need th
shorter each must be . · e
CREATING A RELATIVE FILE
. When a relative file is to be used for the first time, its Open statement will create the
file,
af~e~
that, the same Open statement will be used to re-open the file for both read·
and wntmg . ing
FORMAT STATEMENT TO OPEN A RELATIVE FILE:
OPEN file # , device# , channel # , "drive # : file name, L" + CHR$ (record
length) '
where '. ' file#" is the file number, normally an integer between I and 127; "device# " is
the device numbe.r to be used'.
no~ally
8 on the 1541; "channel #" selects a particular
ch~~~el .along.~~1ch co~municatJOns
for this file can take place, normally between
2 and
14, dn:e # is .the dnve number, always 0 on the 1541 ; and "file name" is the file
?ame, with a maximum length of 16 characters . Pattern matching characters are allowed
'.~
the name
~~~n acc~ssing
an existing file , but not when creating a new one. The
record length. is the size of each record within the file in bytes used, including carriage
returns , quotation marks and other special characters .
56
Notes:
I . Do not precede the drive number with the "at" sign(@ ); there is no reason
to replace a relative file .
2. ,L , "+ CHR$(record length) is only required when a relative file is first
created, though it may used later, so long as the "record length" is the same as
when the file was first created . Since relative files may be read from or written to
alternately and with equal ease, there is no need to specify Read or Write mode
when opening a relative file.
3. "file#", "device#" and "channel#" must be valid numeric constants,
variables or expressions . The rest of the command must be a valid string literal,
variable or expression .
4. Only I relative file can be open at a time on the 1541, although a sequential
file and the command channel may also be open at the same time.
EXAMPLES:
To create or re-open a relative file named "GRADES" , of record length 100, use
OPEN 2,8,2,"GRADES,L," +CHR$(100)
To re-open an unknown relative file of the user's choice that has already been
created, we could use
200 INPUT''WHICH ALE'';A$
210 OPEN 5,8,5,A$
USING RELATIVE FILES: RECORD#
When a relative file is opened for the first time, it is not quite ready for use . Both to
save time when using the file later, and to assure that the file will work reliably, it is
necessary to create several records before closing the file for the first time. At a minimum,
enough records to fill more than 2 disk sectors (512 bytes) should be written . In practice,
most programs go ahead and create as many records as the program is eventually expected
to
use. That approach has the additional benefit of avoiding such problems as running out
of
room on the diskette before the entire file is completed.
If you simply begin writing data to a just-opened relative file, it will act much like a
lequential file, putting the data elements written by the first Print# statement in Record
II, those written by the second Print# statement in record #2 and so on . (As this
implies, each relative record must be written by a single Print# statement, using embed-
ded
carriage returns within the data to separate fields that will be read in via one or more
Input# statements later.) However, it is far better to explicitly specify which record
~Urnber
is desired via a Record# command to the disk . This allows you to access records
Ill any desired order, hopping anywhere in a file with equal ease. Properly used, it also
avoids a subtle error (bug) common to all Commodore disk drives .
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