Q50946: Can’t Directly Delete a Record from a BASIC Random Access File

Article: Q50946
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S891113-44 B_BasicCom B_GWBasicI B_BasicInt | mspl13_basic
Last Modified: 14-DEC-1989

You cannot directly delete a record from a BASIC random access file to
make the file size decrease. This is a DOS and OS/2 operating system
limitation for the random access method. You can, however, work around
this in several different ways as explained below.

This information applies to GW-BASIC Versions 3.20, 3.22, and 3.23 for
MS-DOS; to Microsoft BASIC Interpreter Version 5.28 for MS-DOS; to
Microsoft QuickBASIC Versions 1.00, 1.01, 1.02, 2.00, 2.01, 3.00,
4.00, 4.00b, and 4.50 for MS-DOS; to Microsoft BASIC Compiler Versions
5.35 and 5.36 for MS-DOS and Versions 6.00, and 6.00b for MS-DOS and
MS OS/2; and to Microsoft BASIC PDS 7.00 for MS-DOS and MS OS/2.

Both DOS and OS/2 support operating system calls that allow
applications to create and manipulate files with random and sequential
access. However, neither DOS nor OS/2 offers a record-deleting
feature, such as a feature that reduces a file's size when a record is
deleted. This limitation can be worked around in several different
ways:

1. The individual elements in a data record can be cleared by setting
   all strings to NULL ("") and all numeric items to zero (0). The
   record will still take up space on the disk. The program can be
   written to detect and skip the record when it is processing the
   data file.

OR

2. All of the records of the old data file can be copied to a new data
   file. As the program is copying the file, it can skip any records
   that were "marked" for deletion, as in step 1 above. The old data
   file can then be deleted, and the new file renamed to the original
   data file name.

OR

3. A separate index file can be created that can be used to track
   deleted records. All processing of the file will use the index file
   to skip the deleted records, and when new records are added, the
   key file can be used to insert new records into the location that
   the deleted records used to occupy.

Microsoft BASIC PDS 7.00 has ISAM file support that does support
DELETE for records of ISAM files. Since ISAM files are allocated in
32K chunks, new space will only be gained if a block of 32K can be
removed. To gain this space you must run ISAMPACK.EXE. ISAM file
support is available for MS-DOS only.