Commodore 1541-II - The Value of Index Files

3030 GOSUB 59990 Check for disk errors
3040 INPUT# 1 ,NA$,SA$,CS$,ZP$ Read in fields
3050 GOSUB 59990
3060 PRINT#15,"P" +CHR$(96 + 3) +
CHR$(RL) + CHR$(RH) Re-position for safety
Here are the lines needed to read back the version with fixed length fields:
3000RH = INT(RE/256)
3010RL = RE-256*RH
3020 PRINT#15,"P" + CHR$(96 + 3) + CHR$
(RL) + CHR$(RH)
3030 GOSUB 59990
3040 INPUT# 1 ,DA$ Read in entire record
3050 GOSUB 59990
3060 PRINT#15,"P" +CHR$(96 + 3) +
CHR$(RL) + CHR$(RH)
3070 NA$ = LEFT$(DA$,27) Split data into fields
3080 SA$ = MID$(DA$,28,27)
3090 CS$ = MID$(DA$,55,23)
3100 ZP$ = RIGHT$(DA$, 10)
This ends our discussion of relative files. A complete "RELATIVE FILE" program,
similar to the examples in this chapter, is included on the Test/Demo diskette.
THE VALUE OF INDEX FILES (ADVANCED USERS)
In the last two chapters we have learned how to use sequential and relative files
separately. But they are often used together, with the sequential file used to keep brief
records of which name in the relative file is stored in each record number. That way the
contents of the sequential file can be read into a string array and sorted alphabetically.
After sorting, a technique known as a binary search can be used to very quickly find an
entered name in the array, and read in or write the associated record in the relative file.
Advanced programs can maintain two or more such index files, sorted in differing ways
simultaneously.
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