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1991-04-27
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APPLICATIONS MAKER - HELP FOR FILE ITEMS
The FILE ITEMS execute operations relevant to the manipulation of
datafiles. They are of seven types: GET RECORD, WRITE RECORD, NEW,
RELATIONAL WRITE, LOAD MARKER and SHOW.
GET RECORD ITEMS: With the GET RECORDs the next available record in
the targeted file will be read into the file's buffer. The data it
contains will then be available for use. If you have loaded no
MARKER FILE these items will read all the records in the file. If
you have used the LOAD MARKER ITEM (below) these items will read the
records in the MARKER FILE, in whatever order they're in.
The GET RECORD ITEMS take a label as an argument. When all the
records have been processed (end of file or marker) the GET RECORD
ITEM will do a forward goto, branching to that label.
WRITE RECORD ITEMS: With the WRITE RECORDs the data in the buffer of
the targeted file will be written to disk into the record from which
it came.
These items differ from the RELATIONAL WRITES in that the write
happens whenever the item is encountered and an entire record is
written instead of a single field.
The use of the WRITE RECORD items can cause trouble if your
application terminates prematurely due to an error. Your file has
been updated and the rest of your application, after the error
occurred, did not happen. So, place WRITE RECORD items as near as
possible to the end of your application, after any possibility of
aborting the application.
NEW ITEMS: With the NEW items a new record in the targeted file is
created. Your user will need to select, or allow the program to
select, a record number for the new record.
The NEW item MUST be used in applications that write new records into
datafiles. Failure to do so will cause the first or current record
in the file to be over-written when the write is executed.
When NEW is used the first available hole in the file (caused by
record deletion) or new disk space for the record is read into the
file's buffer and any data it may contain is replaced by spaces or
zero. Thusly, a NEW RECORD is created.
So, use NEW items at the top of your screen, or at the top of the
segment of your "program" that creates a new record. The NEW item
must be executed before any of the fields in the new record are
filled. Using NEW after some of the fields are filled will WIPE THEM
OUT. Use a WRITE RECORD item when you have placed the necessary data
into the record, to record it to disk.
RELATIONAL WRITE ITEMS: With the RELATIONAL WRITEs the relational
writes stored in memory for the targeted file are written to disk.
These relational writes are created when the WRITE TO ANOTHER
DATAFILE option within data items is used. Unlike the WRITE RECORD
item, these items will not be executed until the RELATIONAL WRITE is
encountered, presumably as the last step in your process, and only a
single field is written as opposed to an entire record. So, if your
application ends with an error, nothing is changed.
If you update file five, field seven, then use that file/field again
before you execute the RELATIONAL WRITE the new value will be used as
the list of relational writes is searched as data is read from disk.
Like the DO items the RELATIONAL WRITEs use memory to store their
file, field, new value and update mode. And they could exhaust
available memory if not properly managed. So, if your application is
cyclic and updates files via these items you should place a
RELATIONAL WRITE item for each file updated near the end of the cycle
to conserve memory. If the application bombs these writes will not
be executed, and you're in trouble again.
RESET FILE ITEMS: With the RESET FILEs the target file will be reset
to the first record. If you have used the LOAD MARKER FILE item to
load a marker for the file the marker file will be closed, and you
will be able to load another marker or process all records. It is
not necessary to use this item just to close a marker.
LOAD MARKER ITEMS: With the LOAD MARKERs a MARKER FILE written by
the BASE program in the targeted file is loaded. These MARKER FILES
give you the ability to process a pre-defined group of records in a
pre-defined order. After executing a LOAD MARKER the GET RECORD
items will fetch the next record from the marker instead of the file.
When initializing a LOAD MARKER item you may select a MARKER FILE
from the menu of the markers present. You may also press escape to
bypass file selection. You will then have the option to allow the
user of your application to select the a marker from that same menu.
This is handy to do a report or operation on a group of records
selected at runtime.
SHOW ITEMS: With the SHOW items the currently loaded record in the
targeted file is displayed. You select the file from the files menu,
and you have the option to allow your user to edit the record and
write it back to disk, or to display it read-only.