Floppyshop 2/Floppyshop - 2.zip/Floppyshop - 2.iso/diskmags/3565-4.665/dmg-4385/issue_10/16.pne

Text File | 1987-04-21 | 8KB | 187 lines

                             DISK   ACCESSING
                             ~~~~~~~~~~~~~~~~
                            By  Tony Greenwood
                          (For STOSSER Diskzine)
                          ~~~~~~~~~~~~~~~~~~~~~~

Disk (or is it disc) access is a very important and powerful part of
programming.  Don't you just hate playing a game on your ST for hours and
finally making the highscore table only for it not to be saved to disk and
therefore losing it forever.  I would much prefer to know that the next
person who is going to play will be able to marvel at my high score.  Not
just high score tables can be saved and retrieved from your disk, there
are numerous uses and advantages that can be gained from disk accessing.
The file you are now reading has been accessed from disk of course.
Theres also a save to disk option in the help screen, I am sure you can
think of many more uses.  But do you know how to do it, I think not or you
would'nt be reading this file.  Well i have the problem of not knowing how
much or how little you do know, therefore i will have to start from
scratch and presume you know nothing on the subject.  A little boring for
some of you i know, but at least i know i will have covered all levels of
competence.

Well i have the STOS Manual at my side to refer to.  Its now some time
since i learnt how to use these commands and its only now that i realise
just how useless the manual is.  The commands i first learnt and the ones
we will be covering ie: SEQUENTIAL FILES, "FILENAME.SEQ"

Before i go any further i will stop you falling into the same trap i did
when i was trying to learn these, and that is the fact that the files we
want to access do not have to have SEQ as there extensions.  So we can use
all the following on DOC, TXT, MUS, ASC etc etc...

Right then, the word sequence means that any access to these files will be
done in order from start of file to end of file. In other words if you
wanted to load line 300 from a DOC file you would have to load the first
299 lines to get to it.  Random access files would let you load line 300
on its own, or save line 300 on its own.  In order for us to save one line
we would have to save the whole file, apart from the making of a database
program i can think of no reason to want to load just the one part of a
file.  In fact i have never needed that system so we will just concentrate
on SEQUENCIAL FILES.

                      ~~~~~~~ LOADING ~~~~~~~

First off before we open a file and load it into memory we will need
somewhere to put it.  Lets concentrate on loading a whole doc file into
memory shall we?.  So we would load this doc file line by line, the doc
file could be any legnth could'nt it, so we will DIM an array of 1000.
This will then enable us to load any doc file that is 1000 lines long or
less.  If you think you may need bigger then simply DIM a bigger array of
course, so we are off!.

 10 DIM TXT$(1000)

Next we need to find a file to open, so we will use the file selector to
find one.  You will find the instructions for this in page 218 of your
manual.  But i have just looked at it and its not very clear is it, here
i will show you what to do.

 20 F$=FILE SELECT$ ("*.*")

This will bring up a file selector that will list all the files and
folders in the root directory.  The "*" simply mean show all, so it will
show all the file names and all the extensions.  If you only wanted DOC
files to be listed then you would change it to:

 20 F$=FILE SELECT$ ("*.DOC")

Then whatever file you click on, F$ will hold the name of that file, so if
you click on READ_ME .DOC , then thats what F$ would equal.

Next we want to open that file, we can have more than one file open at
once, so we have to keep them numbered.  We then use that number for
everything we do to that file so as not to confuse STOS.  If you are only
using one file at once then you must still number it.  So to keep things
as clear as possible we will call this one number one, but it must be
prefixed by a #,
So we are working on file #1

 30 OPEN IN #1, F$

"Open" speaks for itself ( i hope ) ,"F$" holds the filename,"#1"  is the
number of the file we are using, "IN" means we are opening the file for
retrieving/getting data and bringing it into our program.  The oposite
function ie: saving data is the opposite command ie: open out #1, f$, but
dont worry about that yet, so long as you fully understand line 30 and
what each peice means.

Now we obviously want more than one peice of information bringing in, ie:
each line of the DOC file is a seperate peice of info that we will be
puttting into each cell of our array.  Therefore we will now need to set
up a loop in order to bring it all in.

 40 E=0 : REPEAT
 50 LINE INPUT #1,TXT$(E)
 60 INC E
 70 UNTIL EOF(#1)
 80 DEC E
 90 CLOSE #1 : rem we have got it!

 Now to explain the above,

Line 40 sets E to 0 as that is the pointer to tell the computer where to
put the info or which part of the array to put it in.  Then REPEAT starts
the loop.

Line 50 gets the first line of the doc we are loading and puts it into
TXT$(0)

Line 60 of course increases E

Line 70 is the end of the loop and checks for the last peice of
information in the document we are loading , or End Of File EOF and the
number of the file, So EOF(#1) checks for end of file one,

If it is the end then we goto line 80, as we have been increasing E after
we have put our information in it.  This means that currently TXT$(E) has
nothing in it, so we dec E to go back to the last peice of information
that we loaded.

Line 90 then closes the file we have been using, ie: #1, In all your
routines.  If you have an OPEN Then you must have a close,


                    ~~~~~~~~~ SAVING ~~~~~~~~~

Well this bit should be a lot shorter than the above as it is quite simple
now we have covered the main points of disk accessing, we are quite simply
going to do the opposite of the loading instructions, let me explain..

To open a file for Saving something to disk we use the following command.

 OPEN OUT #1, "FILENAME.DOC"

Or if you have just loaded a doc file or an high score table or something
with a variable as in the loading commands, open in #1, F$, then you can
use the same variable

OPEN OUT #1,F$

Then instead of INPUT #1, TXT$(E) we would do the opposite and..

PRINT #1, TXT$(E)

So to save out the doc file you loaded in with the loading commands, you
would now use the following.

 100 OPEN OUT #1,F$
 110 FOR N=0 TO E
 120 PRINT #1,TXT$(N)
 130 NEXT N
 140 CLOSE #1

The EOF(#1) will not work when saving out, but we already know how many
lines to save as we got that with the E variable, so its easy enough to
make a controlled loop from 0 to E and we cannot miss then can we.

 Please note : When you open out a filename, it will erase any file with
that filename thats on the disk.  Therefore if you had a doc file with
lets say five hundred lines in it and you wanted to add one line, you
cannot open the file and print to disk just line TXT$(501) ,because then
the file with that filename will then simply consist of the one line you
just saved.  Hence the explanation earlier about SEQUENCE, you would have
to load the whole five hundred lines, then add one line, then save the
five hundred and one lines.

The use of RANDOM ACCESS files will however let you load or save just one
line, but i dont know enough on random access files to enable me to write an article on the subject, If you do then
write it up for us!.

Well i hope the above is enough to get you started on using the saving and
loading commands that are at your disposal.  There are lots of other
commands that can be used in conjunction with the above.  I suggest that
once you can do the above then the rest will come to you a lot easier with
the use of your trusty manual.

 HAPPY STOSSING!