Demonstration Of Sequential Files; Chapter 6: Relative Data Files; Files, Records, And Fields - Commodore 1541 User Manual

DEMONSTRATION OF SEQUENTIAL FILES
Use the following program for your first experiments with sequential files.
Comments have been added to help you better understand it.
150 CR$ = CHR$, ( 13)
160 OPEN 15,8,15
170 PRINT CHR$(147):REM CLEAR
SCREEN
190 PRINT"** WRITE A FILE** "
210 PRINT
220 OPEN 2,8,2, " @O:SEQ FILE,S,W"
230 GOSUB 500
240 PRINT" ENTER A WORD,
THEN A NUMBER' '
250 PRINT"OR 'END,O' TO STOP"
260 PRINT
270 INPUT A$,B
280 PRINT#2,A$;CR$;B
290 GOSUB 500
300 IF A$ <>" END " THEN 270
310 PRINT
320 CLOSE 2
340 PRINT"** READ SAME FILE
BACK**"
360 PRINT
370 OPEN 2,8,2, "O:SEQ FILE,S,R"
380 GOSUB 500
390 INPUT#2 ,A$ ,B
400 RS=ST
410 GOSUB 500
420 PRINT A$ ,B
430 IF RS = 0 THEN 390
440 IF RS <>64 THEN
PRINT' ' STATUS = '';RS
450 CLOSE 2
460 END
480 REM ** ERROR CHECK SIR **
500 INPUT#l5 ,EN,EM$ ,ET,ES
510 IF EN>O THEN PRINT
EN ,EM$,ET ,ES :STOP
520 RETURN
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Make a carriage return variable
Open demo file with replace
Check for disk errors
Accept a string & number
from keyboard
Write them to the disk file
Until finished
Tidy up
Reopen same file for reading
Read next string & number from file
Remember file status
Display file contents
until done,
unless there's an error
Then quit
A Basic 3.5-only version could
replace line 500 with
'500 IF DS>O THEN PRINT
DS$:STOP
and delete line 510
CHAPTER6
RELATIVE DATA FILES
fJ1E
VALUE OF RELATIVE ACCESS
Sequential files are very useful when you 're just working with a continuous stream of
data-
i.e., information that can be read or written all at once . However, sequential files
are
not useful or desirable in some situations . For example , after writing a large list of
mail labels, you wouldn 't want to have to re-read the entire list each time you need a
person's record . Instead, you need some kind of random access, a way to get to a
particular label in your file without having to read through all those preceding it first.
As an example, compare a record turntable with a cassette recorder. You have to
listen to a cassette from beginning to end, but a turntable needle can be picked up at any
time.
and instantly moved to any spot on the record . Your disk drive works like a turntable
in that respect. In this chapter we will learn about a type of file that reflects this flexibility .
Actually , two different types of random access files may be used on Commodore
disk drives: relative files and random files . Relative files are much more convenient for
most data handling operations, but true random access file commands are also available to
advanced users, and will be discussed in the next chapter.
FILES, RECORDS, AND FIELDS
When learning about sequential files, we did not worry about the organization of data
within a file, so long as the variables used to write the file matched up properly with those
which read it back into the computer. But in order for relative access to work, we need a
more structured and predictable environment for our data.
The structure we will use is similar to that used in the traditional filing cabinet . In a
traditional office , all customer records might be kept in a single file cabinet. Within this
file, each customer has a personal record in a file folder with their name on it , that
contains everything the office knows about that person . Likewise, within each file folder,
there may be many small slips of paper, each containing one bit of information about that
customer, such as a home phone number, or the date of the most recent purchase.
In a computerized office, the file cabinet is gone, but the concept of a file containing
all the information about a group or topic remains . The fiie folders are g1,; m e too , but the
notion of subdividing the file into individual records remains. The slips of paper within
the
personal records are gone too, replaced by subdivisions within the records, called
fields. Each field is large enough to hold one piece of information about one record in the
file. Thus, within each file there are many records, and within each record there are
typically many fields .
A relative file takes care of organizing the records for you , numbering them from I to
Whatever, by ones, but the fields are up to you to organize . Each record will be of the
same
size, but the 1541 won't insist that they all be divided the same way . On the other
hand, they normally will all be subdivided the same way , and if it can be known in
advance exactly where each field starts within each record, there are even fast ways to
ilCcess a desired field within a record without reading through the other fields . As all of
lhis implies, access speed is a primary reason for putting information into a relative disk
file. Some well-written relative file programs are able to find and read the record of one
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