File: Varray,		Node: Varray,		Up: (Basic)top





     Virtual memory arrays provides high-speed, random access to files. The
data is stored in a disk file, instead of the memory of the computer, so the
user can quickly retrieve data. The principles of Virtual Array storage is
similar to Record I/O in that the data in the disk file is in a contiguous
sequence of 512-byte records.






          CREATING A VIRTUAL ARRAY DATA FILE

     When creating a virtual memory file, the user must first associate a
disk file name with a channel designator from 1 to 12. This will connect
the program to a disk file and all statement associated with this file must
have this number. The statement that makes this connection is the OPEN
statement. When creating a file, the user must open the file for output which
means that data will be moved from the program's local storage to the file.
For example:

           OPEN "FILE.DAT" FOR OUTPUT AS FILE #1%

     This statement will delete an existing file with the same name and
extension and create a new file.

     To have a matrix of data in virtual memory, the user must declare it in
a special form of the DIM statement. This special DIM statement has the
following format:

          DIM #<file channel number>,<list>

     The file channel number must corresponds to the internal file designator
where the program has opened a disk file. The variable appears as it would in a
DIM statement for a matrix in main memory.
For example:

          DIM #1%, A$(50%,3%)

      Which set-up an array of 50 rows and 3 columns. (Actually 51 rows and 4
columns. There is a row 0 "zero" and a column 0. This column and row can be
used to store special information in the array.)
     The user can also store floating-point constants, integer constants, and
strings in virtual memory matrices. The user can also specify more than one
matrix in a virtual memory file. For example:

          DIM #7%, A(100%), B%(50%), C$(25%)

      One of the problem a user might have when using virtual array storage is
the storage of string variables. If the user does not specify a length, the
system assumes a default length of 16 characters. However there are times when
storing string variables that the length needs to be longer than 16 characters.
So the program can define the maximum length, which varies from 2 to 512
characters which is the power of 2. (i.e. 2,4,8,16,32,64,128,256,512)
Thus:

          DIM #4%, X$(10%)=128

     Where the maximum string length will be 128 characters.

     The user should extend the file to its file length when creating the
file. Because of the way this system stores the data, it is more efficient
to extend the file to it's maximum file size. For example, if the user knows
that up to 100 rows of data is needed for array A$, but the user has only
10 rows of data to start with, the user should dimension array A$ to have 100
rows when creating the file.


     A simple statement disconnects the channel between the program and the
file. It's called the CLOSE statement. The user must use this statement to
make sure the file is closed properly.
For example:

          CLOSE #1%










          10  OPEN "FILE.DAT" FOR OUTPUT AS FILE #1%
          20  DIM #1%,D$(50,3)
          30  INPUT "NUMBER OF EMPLOYEES",E
          40  D$(0,0)=NUM$(E)
          50  FOR I%=1% TO E
          60  INPUT "NAME",D$(I%,1%)
          70  INPUT "ADDRESS",D$(I%,2%)
          80  INPUT "SALARY",S
          90  D$(I%,3%)=NUM$(S)
          100 NEXT I%
          110 CLOSE #1%
          120 END











          10  OPEN "FILE.DAT" FOR INPUT AS FILE #7%
          20  DIM #7%,D$(50%,3%)
          30  E=VAL(D$(0,0))
          40  PRINT TAB(30);"EMPLOYEE REPORT"
          50  PRINT" NAME         ADDRESS              SALARY"
          60  Z$=  " \         \  \                \   $$###.##"
          70  FOR I%=1% TO E
          80  S=VAL(D$(I%,3%))
          90  PRINT USING Z$,D$(I%,1%),D$(I%,2%),S
          100 NEXT I%
          110 CLOSE #7%
          120 END











          10  OPEN "FILE.DAT" AS FILE #10%
          20  DIM#10%, D$(50%,3%)
          25  E=VAL(D$(0,0))
          30  PRINT"CHOOSE ONE OF THE FOLLOWING"
          40  PRINT " (1) CHANGE ADDRESS"
          50  PRINT " (2) CHANGE SALARY"
          60  PRINT " (3) TO END"
          70  INPUT "INPUT 1-3",A%
          80  IF A%<1% OR A%>3% THEN PRINT"INVALID NUMBER"\RESUME 70
          90  IF A%=3% GO TO 400
          100 INPUT "INPUT NAME";N$
          110 FOR I%=1% TO E
          120   IF N$=D$(I%,1%) GO TO 150
          130 NEXT I%
          140 PRINT "INVALID NAME"\RESUME 30
          150 ON A% GO TO 200,300
          200 INPUT "INPUT NEW ADDRESS",D$(I%,2%)
          210 GO TO 30
          300 INPUT "INPUT NEW SALARY",S
          310 D$(I%,3%)=NUM$(S)
          320 GO TO 30
          400 CLOSE #10%
          410 END