How to Remove Records from a Table Without Using Delete/Pack

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for MS-DOS, versions 2.0, 2.5, 2.5a, 2.6, 2.6a
• Microsoft FoxPro for Windows, versions 2.5, 2.5a, 2.5b, 2.6, 2.6a

SUMMARY

Using the DELETE or PACK commands to remove records can be very time consuming. ZAP is very fast, but removes all the records. With FoxPro's low-level file I/O, you can quickly remove a select number of unwanted records from the end of a table.

MORE INFORMATION

Step-by-Step Procedure

1. Open a program file, name it Lowlevel.prg, and type the following function code:

   
      FUNCTION ZAPLAST
      PARAMETER lnY
      IF USED()
        lnZ=RECCOUNT()
        IF lnY<=lnZ
          lcFILENAME=DBF()
          USE
          lnHANDLE=FOPEN(lcFILENAME,2)
          IF lnHANDLE>0
            lcBYTE=FREAD(lnHANDLE,32)
            lnFLD_SIZE=ASC(SUBSTR(lcBYTE,11,1))+;
               (ASC(SUBSTR(lcBYTE,12,1))*256)
            lnFILESIZE=FSEEK(lnHANDLE,0,2)
            lnX=lnZ-lnY
            lcBYTE8=CHR(INT(lnX/(256*256*256)))
            lcBYTE7=CHR(INT(lnX/(256*256)))
            lcBYTE6=CHR(INT(lnX/256))
            lcBYTE5=CHR(MOD(lnX,256))
            lcBYTE=SUBSTR(lcBYTE,1,4)+lcBYTE5+;
               lcBYTE6+lcBYTE7+lcBYTE8+SUBSTR(lcBYTE,9)
            =FSEEK(lnHANDLE,0)
            =FWRITE(lnHANDLE,lcBYTE)
            =FCHSIZE(lnHANDLE,lnFILESIZE+(lnFLD_SIZE*lnX))
            =FCLOSE(lnHANDLE)
          ENDIF
          USE &lcFILENAME
        ENDIF
      ENDIF 




2. Save and close Lowlevel.prg



3. Test the function by removing a single record from a table. For this example, use the Customer.dbf table - but back up the table first. Records that are deleted with ZAPLAST() cannot be recovered.



1. In the Command window, type:

   
         SET PROCEDURE TO LOWLEVEL.PRG
         USE C:\FPW26\TUTORIAL\CUSTOMER EXCLUSIVE 




2. Take note of how many records are in the table. For this example, there are 500 records.



3. In the Command window, type:

   
         =ZAPLAST(1) 




4. Note that there is one less record in the database. Originally there were 500 records; now there are only 499.



4. Any number of records can be deleted. For example, to delete 27 records, change step 3.c. to =ZAPLAST(27)

Additional query words: VFoxWin FoxWin FoxDos

Keywords :
Version :
Platform :
Issue type :