Can't Directly Delete a Record from a Basic Random Access File (50946)


The information in this article applies to:

  • Microsoft GW-BASIC 3.2
  • Microsoft GW-BASIC 3.22
  • Microsoft GW-BASIC 3.23
  • Microsoft GW-BASIC 5.28
  • Microsoft QuickBASIC 4.0
  • Microsoft QuickBASIC 4.0b
  • Microsoft QuickBASIC 4.5
  • Microsoft BASIC Compiler for MS-DOS and OS/2 6.0
  • Microsoft BASIC Compiler for MS-DOS and OS/2 6.0b
  • Microsoft Basic Professional Development System (PDS) for MS-DOS and MS OS/2 7.0
This article was previously published under Q50946

SUMMARY

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.

MORE INFORMATION

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.
Modification Type:MinorLast Reviewed:8/16/2005
Keywords:KB50946
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