QDOS SuperBASIC File Exists Function

[bwinkel67]

Without adding any toolkits to QDOS, what's the simplest way of checking if a file exists in SuperBASIC? I came up with a method that opens a file, dir's into that file, the loops through it to compare file names, but I was wondering if there was a simpler way to do that (similar to what FTEST from TK2 does).

[Andrew]

I prefer to use Toolkit 2 functions, but without that I would probably use something like

Code: Select all

100 WHEN ERRor
110   IF ERR_NF
120      PRINT#0,'File does not exist!'\'Press a key'
130      RETRY 500
140   END IF
150 END WHEN
500 INPUT  "Filename:  ";fn$
510 OPEN_IN#4, fn$
520 PRINT#0,'File '&fn$&' exists!': close#4

Of course, you should also check for ERR_IU (IN USE -9), ERR_FE (FILE ERROR -16) and maybe other errors.

[dilwyn]

Another way is to error trap an OPEN_IN statement on ROMs which implement WHEN ERRor (i.e. not versions AH, JM or earlier). Sadly, even on ROMs which do implement WHEN, that's not always 100% reliable in interpreted BASIC apart from Minerva, though it should be OK when compiled on all ROMs.

As you've realised, doing this on a QL without Toolkit 2 can be fiddly and rather long-winded. A QL without TK2 feels rather bare these days!

EDIT:Sorry Andrew, our replies crossed

[Derek_Stewart]

Without adding any toolkits to QDOS, what's the simplest way of checking if a file exists in SuperBASIC? I came up with a method that opens a file, dir's into that file, the loops through it to compare file names, but I was wondering if there was a simpler way to do that (similar to what FCHECK from TK2 does).

Hi,

Do you mean the Toolkit 2 function: FTEST

I see no mention of FCHECK in Toolkit 2, Superbasic Online Manual, other SBASIC texts. Can you explain where the FCHECK command / function comes from and what is the syntax?

[bwinkel67]

Do you mean the Toolkit 2 function: FTEST

Yes, FCHECK was a typo (misremembering).

[bwinkel67]

Oh, I like the WHEN ERRor method. I ended up writing a long-winded function and was hoping that either a command already existed or it could be more efficient. I think the OPEN_IN/WHEN ERRor method might just do that.

Here is my FCHECK function (that's why I confused the name with TK2's FTEST). This one doesn't use WHEN, so if that's flaky, then maybe this will do. Of course it requires writing to a disk but if a dynamic ram disk were installed, then it could use that.

Code: Select all

100 DEFine FuNction FCHECK(mdv$,name$)
105   DELETE mdv$&"dir"
110   OPEN_NEW #9, mdv$&"dir"
115   DIR #9, mdv$: CLOSE #9
120   OPEN_IN #9,mdv$&"dir"
125   REPeat loop
130     IF EOF(#9): CLOSE #9: DELETE mdv$&"dir": RETurn 0
135     INPUT #9, file$
140     IF file$=name$: CLOSE #9: DELETE mdv$&"dir": RETurn 1
145   END REPeat loop
150 END DEFine

[Derek_Stewart]

Hi,

Yes, FCHECK was a typo (misremembering).

I thought that I had not read the manual right.

If you are not using any toolkit extensions, then you are going have problems, as the WHEN ERRor error trapping did not really work correctly, till JS, ROM, but still had issues, Minerva fixed it.

But maybe the US ROMs were all debugged.

[tofro]

Oh, I like the WHEN ERRor method. I ended up writing a long-winded function and was hoping that either a command already existed or it could be more efficient. I think the OPEN_IN/WHEN ERRor method might just do that.

Here is my FCHECK function (that's why I confused the name with TK2's FTEST). This one doesn't use WHEN, so if that's flaky, then maybe this will do. Of course it requires writing to a disk but if a dynamic ram disk were installed, then it could use that.

Code: Select all

100 DEFine FuNction FCHECK(mdv$,name$)
105   DELETE mdv$&"dir"
110   OPEN_NEW #9, mdv$&"dir"
115   DIR #9, mdv$: CLOSE #9
120   OPEN_IN #9,mdv$&"dir"
125   REPeat loop
130     IF EOF(#9): CLOSE #9: DELETE mdv$&"dir": RETurn 0
135     INPUT #9, file$
140     IF file$=name$: CLOSE #9: DELETE mdv$&"dir": RETurn 1
145   END REPeat loop
150 END DEFine

One proposal: Instead of writing to a temporary file on disk, I would use a pipe to "DIR" into and read from. That eliminates most of the mess the function might leave behind. Pipes in QDOS are extremely underrated.

[bwinkel67]

One proposal: Instead of writing to a temporary file on disk, I would use a pipe to "DIR" into and read from. That eliminates most of the mess the function might leave behind. Pipes in QDOS are extremely underrated.

I understand pipes exist in QDOS but are they exposed to the user in SuperBASIC? Could you show me an example of how to do it as that would be pretty neat...

NOTE if you didn't read the first post, I'm trying to do this in JS ROM with no add-on toolkits, so no TK2.

[swensont]

IIRC, Pipes or Filters only work with TKII. I checked the TKII manual and it shows an example of pipes using the EX command:

EX [Chan # TO] prog_spec {TO prog_spec} [ TO Chan #]

Tim