Expanding A Relative File - Commodore 1541 User Manual

1410 OPEN 1,8 ,2, " O:"+ FI$+" ,L," +CHR$(RL)
1420 GOSUB 59990
1430 RH= lNT(NR/256)
1440 RL=NR-256*RH
1450 PRINT#l5, "P" + CHR$(96 + 2) +
CHR$(RL) + CHR$(RH)
1460 GOSUB 59990
1470 PRINT#l,CHR$(255);
1480 GOSUB 59990
1490 PRINT#l5 , "P"+CHR$(96+2)+
CHR$(RL) + CHR$(RH)
1500 GOSUB 59990
1510 CLOSE I
1520 GOSUB 59990
9980 CLOSE 15
9990 END
59980 REM CHECK DISK SUBROUTINE
59990 INPUT#l5,EN ,EM$,ET,ES
Begin to create desireQ
Check for disk errors
Calculate length
VaJu
Cs
Position to last record
number
Send default character to
Re-position for safety
Now the file can
be
sai
closed
And the command
eh
closed
Before we end the
pro.
gram
G RELATIVE FILE DATA
1be
commands used to read and write relative file data are the same Print# , Input# ,
(jtl.# commands used in the preceding chapter on Sequential files. Each command is
,J
15
described there . However, some aspects of relative file access do differ from
~al
file programming , and we will cover those differences here .
AS
seated earlier in this chapter, each relative record has a fixed length , including all
·ial
characters . Within that fixed length, there are two popular ways to organize
~oUS
individual fields of information . One is free-format, with individual fields varying
i0gth
from record to record , and each field separated from the next by a carriage return
(each of which does take up 1 character space in the record) . The other approach
~ use
fixed-length fields, that may or may not be separated by carriage returns . If fixed
fields are not all separated by carriage returns, you will either need to be sure a
return is included within each 88 character portion of the record . If this is not
' O C.
you
will have to use the Get# command to read the record , at a significant cost in
Relative records of 88 or fewer characters, or final portions of records that are 88 or
0
ercharacters in length, need not end in a carriage return . :fhe 1541 is smart enough to
·ognize the end of a relative record even without a final• carriage return . Though the
1 1ng of
a single character isn't much, when multiplied by the number of records on a
60000 IF EN > 1 AND EN<>50 THEN PRINT
EN ,EM$,ET ,ES :STOP Ignore "RECORD
NOT
ette, the savings could be significant.
PRESENT" Since each relative record must be written by a single Print# statement , the recom-
60010 RETURN
Two lines require additional explanation . When line 1470 executes, the disk drive
(
operate for up to ten or more minutes , creating all the records in the file , up to
maximum record number you selected in line 1390. This is normal , and only needs to
done once . During the process you may hear the drive motor turning and an occasi
slight click as the head steps from track to track , everything is probably just fine . Sec
ndcd
approach is to build a copy of the current record in memory before writing it to
it
It can be collected into a single string variable with the help of Basic ' s many string-
. g functions, and then all written out at once from that variable .
Here is an example . If we are writing a 4-line mail label , consisting of 4 fields named
AME",
"STREET" , "CITY & STATE", and " ZIP CODE ", and have a total record
of87 characters, we can organize it in either of two ways:
line 60000 above is different from the equivalent line in the error check
subroutine gi
1
earlier. Here disk error number 50 is specifically ignored , because it will be gene. Id N
when the error channel is chec;ked in line 1460. We ignore it because not havtn! ame
Length
WITH VARIABLE LENGTH FIELDS
Field Name Length
requested record would only be ail error if that record had previously been created.
\ME
27 characters NAME 3 1 characters
EXPANDING A RELATIVE FILE
What if you underestimate your needs and need to expand a relative fil e later?
problem . Simply request the record number you need, even if it doesn't currently
e~ 1
the file . If there is no such record yet,
oos·
will create it as soon as you try to
~
information in it , and also automatically create any other missing records beloW
11
number. The only penalty will be a slight time delay while the records are created.
60
27 characters STREET 3 1 character
23 characters CITY & STATE 26 characters
I 0 characters ZIP CODE 1 I characters
87 characters Potent ial length
99 character
Edited length
87 charac ter
With fixed length records. the fi eld lengths add up to exactly the record length . Si nce
·
1
otal
length is just within the Input buffer size limitation, no carri age return charac ters
: ileeded. With variable length records, we can take advantage of the vatiability of
<Ja! address lengths . While one name contains 27 letters. another may have onl y 15,
.the
same variability exists in Street and City length s. Al though vari able length record
1
character per fie ld for carriage returns, they ca n take advantage of the difference
61