Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

Kay C Lan wrote:

On Sat, Feb 13, 2010 at 8:25 AM, Alex Tweedly  wrote:


Could you try this in your multi-line msg box and let me know what it does
on Mac ?

put "abcdefg" & CR into URL "file:b.txt"
put URL "file:b.txt"
open file "b.txt" for append
write "x" to file "b.txt"  at 2
close file "b.txt"
put URL "file:b.txt" after msg

on Win, it produces

abcdefg
abx

Alex,

on OS X.6.2, MBP, Rev 4.0

abcdefg
abcdefg
x


I get what Kay got on OS X 10.5.8, which is the correct behavior. Append 
is supposed to force any writes to the end of the existing file. I 
tested some more with the "open for write" command, which should do what 
Alex was after -- truncating the text at the character offset, and 
adding any new data at that position. This fails on both Windows and Mac.


Bug report has been submitted. For details, see:


--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

Kay C Lan wrote:

On Sat, Feb 13, 2010 at 8:25 AM, Alex Tweedly  wrote:


Could you try this in your multi-line msg box and let me know what it does
on Mac ?

put "abcdefg" & CR into URL "file:b.txt"
put URL "file:b.txt"
open file "b.txt" for append
write "x" to file "b.txt"  at 2
close file "b.txt"
put URL "file:b.txt" after msg

on Win, it produces

abcdefg
abx

Alex,

on OS X.6.2, MBP, Rev 4.0

abcdefg
abcdefg
x


I did some quick tests and it looks like the write command is broken on 
OS X. Endian issue with Intel Macs maybe. Don't open a file "for write" 
until they fix it, you can lose data. I'll file a bug report.


The reason I just noticed this is because I was testing some of these 
suggestions. The version of "open file" we should be using, the one 
which truncates contents, is the "open for write" command. However, no 
matter at what position I write to the file, the entire contents were 
replaced. Don't go there if you're on a new Mac.


You won't lose data with other types of open file, but the write 
position may be off. It sounds like it works okay on Windows, and I 
suspect Linux is okay too. I remember using "open file for write" on my 
PPC machine and had no trouble, which is why I suspect an Intel issue.


--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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Re: Reading/Deleting Last Line Of File

[Kay C Lan]

On Sat, Feb 13, 2010 at 8:25 AM, Alex Tweedly  wrote:

> Could you try this in your multi-line msg box and let me know what it does
> on Mac ?
>>
>> put "abcdefg" & CR into URL "file:b.txt"
>> put URL "file:b.txt"
>> open file "b.txt" for append
>> write "x" to file "b.txt"  at 2
>> close file "b.txt"
>> put URL "file:b.txt" after msg
>
> on Win, it produces
>>
>> abcdefg
>> abx
>
Alex,

on OS X.6.2, MBP, Rev 4.0

abcdefg
abcdefg
x

:-(
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Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

J. Landman Gay wrote:

Alex Tweedly wrote:


I strongly suspect it only works on Windows :-(   Sorry.
I don't have either a Mac or Linux box to try it on, however I did 
just try it on the on-rev server (as an .irev script) and it also 
appears to leave the file unchanged there.


I wonder if it has to do with the OS-dependent line-ending conversion 
that the engine does when it writes to text files. If you open the file 
as binfile instead of plain "file" you'll avoid that conversion. It 
might work differently that way.


I'll withdraw this, it's wrong. Whatever OS your script is on, the file 
it writes will will have matching line endings. So nope.


If I finish all the other stuff I need to do this weekend I'll try some 
of the tests everyone posted.


--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

Alex Tweedly wrote:


I strongly suspect it only works on Windows :-(   Sorry.
I don't have either a Mac or Linux box to try it on, however I did just 
try it on the on-rev server (as an .irev script) and it also appears to 
leave the file unchanged there.


I wonder if it has to do with the OS-dependent line-ending conversion 
that the engine does when it writes to text files. If you open the file 
as binfile instead of plain "file" you'll avoid that conversion. It 
might work differently that way.




Playing around some more, it appears that the results of write to file 
in these odd cases are not always what you'd expect.


I would have expected the following two fragments to be equivalent


seek to 2 in file "b.txt"
write "x" to file "b.txt"

and


write "x" to file "b.txt" at 2


That looks right. The "write" command always begins writing at the 
postion after the pointer. But when you specify an exact postion in 
particular, it writes where you tell it.


--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Alex Tweedly wrote:

> I suspect the *trick* of opening a file for append, then seeking back
> into the middle of the file before doing a write is OS-dependent. It's
> certainly not documented (either way), and I was surprised when Jim
> said
> that it could be done.
> Intuitively, 'append' should mean that the existing file is unchangable
> - all you can do is add to it.
> 
> Playing around some more, it appears that the results of write to file
> in these odd cases are not always what you'd expect.
> 
> I would have expected the following two fragments to be equivalent
> 
> > seek to 2 in file "b.txt"
> > write "x" to file "b.txt"
> and
> 
> > write "x" to file "b.txt" at 2
> to be equivalent, but they are not. The latter truncates the file after
> the write, but the former does not (in Windows).

I have never used the *seek* that both you and Richard had in your code
examples for write. I've never even used seek for a read, preferring to keep
track of offsets myself. This is why, in the example that I gave back on the
9th (and tested at the time), I used *open for append - write at offset -
close*. I also have never written an empty, but some truncated data. 

So, I just assumed and it sounded "logical" that seeking and then writing an
empty was the equivalent of what I knew worked. I guess not. Good to know.

In any case, I do know that if you open for append and write at least one
char at an offset, it does truncate the file at the end of that char. And I
know this works on at least PC and Mac.

Aloha from Hawaii,

Jim Bufalini

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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Richard Gaskin wrote:

Jim Bufalini wrote:


Richard Gaskin wrote:


But FWIW, I tried your version and it seemed to leave the file
unchanged


If your original file ends in a cr then Alex's code would end in 
absolutely

no change. ;-)


Thanks, Jim.  I tried it both ways; no change to the file's contents, 
although the mod date gets updated so I know it's trying.

I strongly suspect it only works on Windows :-(   Sorry.
I don't have either a Mac or Linux box to try it on, however I did just 
try it on the on-rev server (as an .irev script) and it also appears to 
leave the file unchanged there.


In fact, it gives an error ("error in offset expression") on the seek, 
because the test file is less than 1000 chars - an error which didn't 
get flagged up on Windows. I changed the line from

   seek tFileLength-K in file pFile
to
  if tFileLength-k > 0 then seek tFileLength-K in file pFile
to avoid the error. But it then leaves the file unchanged.

I suspect the *trick* of opening a file for append, then seeking back 
into the middle of the file before doing a write is OS-dependent. It's 
certainly not documented (either way), and I was surprised when Jim said 
that it could be done.
Intuitively, 'append' should mean that the existing file is unchangable 
- all you can do is add to it.


Playing around some more, it appears that the results of write to file 
in these odd cases are not always what you'd expect.


I would have expected the following two fragments to be equivalent


seek to 2 in file "b.txt"
write "x" to file "b.txt"

and


write "x" to file "b.txt" at 2
to be equivalent, but they are not. The latter truncates the file after 
the write, but the former does not (in Windows).


Could you try this in your multi-line msg box and let me know what it 
does on Mac ?

put "abcdefg" & CR into URL "file:b.txt"
put URL "file:b.txt"
open file "b.txt" for append
write "x" to file "b.txt"  at 2
close file "b.txt"
put URL "file:b.txt" after msg

on Win, it produces

abcdefg
abx

Thanks
-- Alex.







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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Jim Bufalini wrote:

Richard Gaskin wrote:

  

But FWIW, I tried your version and it seemed to leave the file
unchanged



If your original file ends in a cr then Alex's code would end in absolutely
no change. ;-)
  
That's not what my testing showed (or appeared to show). Here's the 
short version of the test script, and the corresponding output
note the first example has a single final CR, the second has no CR and 
the third one has multiple CRs, and each is correctly truncated.


(I missed a few test cases here - empty file, single line of data, etc.  
but it is only "reasonably  tested")


The test stack can be downloaded from 
http://www.tweedly.org/deletelastline.rev
(btw - I don't currently have access to a mac, so it's only tested on 
Windows. Could that cause the different result for Richard ?)



on mouseUp
  
   put empty into field "F"

   put "abcdefg" & CR into t
   put "1234567" & CR after t
   put t into URL ("file:a,b.txt")
   put "abcdefg" & CR into t
   put "1234567"  after t
   put t into URL ("file:b.txt")
   put "abcdefg" & CR into t
   put "1234567" & CR &CR after t
   put t into URL ("file:c.txt")
   putfile "a,b.txt"
   deletelastline "a,b.txt"
   putfile "a,b.txt"

   putfile "b.txt"

   deletelastline "b.txt"
   putfile "b.txt"
   
   putfile "c.txt"

   deletelastline "c.txt"
   putfile "c.txt"
end mouseUp
 
on putfile p

   put "file " && p & CR after field "F"
   put URL ("file:" & P) after field "F"
   put "endoffile" & CR after field "F"
end putfile
 

giving


file  a,b.txt
abcdefg
1234567
endoffile
file  a,b.txt
abcdefg
endoffile
file  b.txt
abcdefg
1234567endoffile
file  b.txt
abcdefendoffile
file  c.txt
abcdefg
1234567

endoffile
file  c.txt
abcdefg
1234567
endoffile


-- Alex.
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Re: Reading/Deleting Last Line Of File

[Richard Gaskin]

Jim Bufalini wrote:


Richard Gaskin wrote:


But FWIW, I tried your version and it seemed to leave the file
unchanged


If your original file ends in a cr then Alex's code would end in absolutely
no change. ;-)


Thanks, Jim.  I tried it both ways; no change to the file's contents, 
although the mod date gets updated so I know it's trying.


--
 Richard Gaskin
 Fourth World
 Rev training and consulting: http://www.fourthworld.com
 Webzine for Rev developers: http://www.revjournal.com
 revJournal blog: http://revjournal.com/blog.irv
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Richard Gaskin wrote:

> But FWIW, I tried your version and it seemed to leave the file
> unchanged

If your original file ends in a cr then Alex's code would end in absolutely
no change. ;-)

Aloha from Hawaii,

Jim Bufalini

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Re: Reading/Deleting Last Line Of File

[Richard Gaskin]

Alex Tweedly wrote:


Richard Gaskin wrote:

That looks similar to what I posted here on the 9th:

   open file tFile for update
   seek relative -1000 in file tFile
   repeat
  read from file tFile until cr
  if it is not empty then
 put it after tBuffer
  else
 delete last line of tBuffer
 write tBuffer to file tFile
  end if
   end repeat
   close file tFile

Does that not do what you need?


No, it doesn't. If a file is opened for 'update' then any write to the
file simply overwrites any existing characters at the appropriates
position(s), and leaves everything following that unchanged. There is no
EOF implied by a write in update mode.


Good catch.  Seems my instinct to use "append" in my first version of 
that was closer to the mark.


But FWIW, I tried your version and it seemed to leave the file unchanged 
- is this a user error on my end, or does it have a limitation I overlooked?


> on deletelastline pFile
>constant K = 1000
>put the detailed files into t
>filter t with URLEncode(pFile) & ",*"
>put item 2 of t into tFileLength
>
>open file pFile for read
>seek to tFileLength-K in  file pFile
>read from file pFile until end
>close file pFile
>
>put the number of chars in the last line of it into tNum
>
>open file pFile for append
>write empty to file pFile at (tFileLength-tNum-3)
>close file pFile
>
> end deletelastline

--
 Richard Gaskin
 Fourth World
 Rev training and consulting: http://www.fourthworld.com
 Webzine for Rev developers: http://www.revjournal.com
 revJournal blog: http://revjournal.com/blog.irv
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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Jim Bufalini wrote:


Just one thing Alex, you need to:

put URLDecode(the detailed files) into t

in case the file name has, for example, a space in it.
  

Thanks Jum, I didn't spot that.

But then, if the file name had a comma in it, I'd be caught out. I think 
what I needed was to URLEncode(pFile) to use in the filter :



on deletelastline pFile
   constant K = 1000
   put the detailed files into t
   filter t with URLEncode(pFile) & ",*"
   put item 2 of t into tFileLength
   
   open file pFile for read

   seek to tFileLength-K in  file pFile
   read from file pFile until end
   close file pFile
   
   put the number of chars in the last line of it into tNum
   
   open file pFile for append

   write empty to file pFile at (tFileLength-tNum-3)
   close file pFile
   
   end deletelastline

-- Alex.
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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Richard Gaskin wrote:

That looks similar to what I posted here on the 9th:

   open file tFile for update
   seek relative -1000 in file tFile
   repeat
  read from file tFile until cr
  if it is not empty then
 put it after tBuffer
  else
 delete last line of tBuffer
 write tBuffer to file tFile
  end if
   end repeat
   close file tFile

Does that not do what you need?

No, it doesn't. If a file is opened for 'update' then any write to the 
file simply overwrites any existing characters at the appropriates 
position(s), and leaves everything following that unchanged. There is no 
EOF implied by a write in update mode. There is an EOF inserted after a 
write if the file is opened in 'append' mode - but then you cannot read 
from it.


(Actually, there is another additional reason - each time you do a 
'read' that updates the file pointer, so the subsequent write would 
begin at the file position immediately *after* the last read, not where 
the read had been done from; you would have needed another seek before 
write  (or do a 'write at x') to have a chance, but you'd still need Rev 
to provide access to truncate() after your write.)


-- Alex.
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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Jeff Massung wrote:


I'm still new to Rev, but in other languages this is dead simple:


  

...

// nuke everything else at the end of the file
trunctate(fp, new_len);

  

...

Done.

Now, maybe this isn't as easy in Rev as it is in C and *many* other
languages. But it should be [if it isn't].
  
That is *exactly* the problem - Rev doesn't expose ftruncate() (or 
ftell() either), so what should be very simple becomes very difficult.


In C, or Python, or even shell it is simple, but Rev's incomplete 
interface to the file system causes the problem.


I opened  a RQCC report  (#1851) in 2004 to request the enhancement of 
providing a way to get the file position (i.e. ftell()). That report 
remains "Unconfirmed".


Given that response, I have not chosen to waste any time entering an 
enhancement request for truncate.


-- Alex.
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Re: Reading/Deleting Last Line Of File

[Richard Gaskin]

Jeff Massung wrote:


I'm still new to Rev, but in other languages this is dead simple:

void truncate_huge_file(const char* filename)
{
FILE* fp = fopen(filename,"wb");
char bytes[200];
size_t new_len;

// move to the end of the file, read 200 bytes
fseek(fp, -200, SEEK_END);
fread(bytes, 1, 200, fp);

// .. TODO: scan for end of line, repeat as needed
// .. TODO: seek SEEK_CUR past what should still be there

new_len = ftell(fp);

// nuke everything else at the end of the file
trunctate(fp, new_len);
fclose(fp);
}

Done.

Now, maybe this isn't as easy in Rev as it is in C and *many* other
languages. But it should be [if it isn't].



That looks similar to what I posted here on the 9th:

   open file tFile for update
   seek relative -1000 in file tFile
   repeat
  read from file tFile until cr
  if it is not empty then
 put it after tBuffer
  else
 delete last line of tBuffer
 write tBuffer to file tFile
  end if
   end repeat
   close file tFile

Does that not do what you need?

--
 Richard Gaskin
 Fourth World
 Rev training and consulting: http://www.fourthworld.com
 Webzine for Rev developers: http://www.revjournal.com
 revJournal blog: http://revjournal.com/blog.irv
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Re: Reading/Deleting Last Line Of File

[Jeff Massung]

On Thu, Feb 11, 2010 at 5:49 PM, Alex Tweedly  wrote:

> Jeff Massung wrote:
>
>> Warren,
>>
>> I've read through most of these suggestions, but I'm surprised that the
>> obvious hasn't been suggested yet (that I've seen): skip everything...
>>
>>
>>
> Hasn't been suggested because it won't work.
>
>
>  put the length of url "file:myfile.txt" into tEnd
>> open file "myfile.txt" for text update
>> seek to tEnd in file "myfile.txt"
>>
>> >From here just back up a "reasonable" number of characters... say 200.
>> Find
>> the last CR character, nuke everything else and close the file. Didn't
>> find
>> one? Try backing up another 200, etc. It'll be a whole lot faster.
>>
>>
>>
> "... nuke everything else " ?? How ?
>
>
I'm still new to Rev, but in other languages this is dead simple:

void truncate_huge_file(const char* filename)
{
FILE* fp = fopen(filename,"wb");
char bytes[200];
size_t new_len;

// move to the end of the file, read 200 bytes
fseek(fp, -200, SEEK_END);
fread(bytes, 1, 200, fp);

// .. TODO: scan for end of line, repeat as needed
// .. TODO: seek SEEK_CUR past what should still be there

new_len = ftell(fp);

// nuke everything else at the end of the file
trunctate(fp, new_len);
fclose(fp);
}

Done.

Now, maybe this isn't as easy in Rev as it is in C and *many* other
languages. But it should be [if it isn't].

Jeff M.
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Alex Tweedly wrote:

> Jeff Massung wrote:
> > Warren,
> >
> > I've read through most of these suggestions, but I'm surprised that
> the
> > obvious hasn't been suggested yet (that I've seen): skip
> everything...
> >
> >
> Hasn't been suggested because it won't work.
> 
> > put the length of url "file:myfile.txt" into tEnd
> > open file "myfile.txt" for text update
> > seek to tEnd in file "myfile.txt"
> >
> > >From here just back up a "reasonable" number of characters... say
> 200. Find
> > the last CR character, nuke everything else and close the file.
> Didn't find
> > one? Try backing up another 200, etc. It'll be a whole lot faster.
> >
> >
> "... nuke everything else " ?? How ?
> You've opened the file for update, so anything that you write to the
> file *overwrites* any existing characters at the same position(s), and
> leaves everything following that unchanged. So there is no way to
> shorten a file if opened in update mode.
> 
> I *think* the best you can do is
> 
> 1. find file length (best to use the detailed files)
> 2. open for read, and read some chunk at the end (200, 1000, whatever
> )
>(open for read is more efficient than open for update)
> 3. calculate the number of bytes to be deleted off the end of the file
> 4. close the file
> 5. open the file for append
> 6. write empty to the file at the appropriate point
> > Edge case to concern yourself with would be if your file happens to
> end with
> > a CR and maybe you want to ignore those cases. But that should be
> easy
> > enough to solve.
> >
> >
> Easiest (and I think best) way to resolve that corner case is to
> preserve the trailing CR if it's there to begin with, and not leave one
> if it wasn't there to start with. This happens to involve no extra work
> at all, hence it's the easiest one :-)
> 
> I decided it was time to email some (reasonably tested) revTalk rather
> than just ideas 
> 
> (NB assumes that the defaultfolder has already been set to the
> appropriate place)
> > on deletelastline pFile
> >constant K = 1000
> >put the detailed files into t
> >filter t with pFile & ",*"
> >put item 2 of t into tFileLength
> >
> >open file pFile for read
> >seek to tFileLength-K in  file pFile
> >read from file pFile until end
> >close file pFile
> >
> >put the number of chars in the last line of it into tNum
> >
> >open file pFile for append
> >write empty to file pFile at (tFileLength-tNum-3)
> >close file pFile
> >
> >end deletelastline
> -- Alex.

Just one thing Alex, you need to:

put URLDecode(the detailed files) into t

in case the file name has, for example, a space in it.

Aloha from Hawaii,

Jim Bufalini

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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

Jeff Massung wrote:

Warren,

I've read through most of these suggestions, but I'm surprised that the
obvious hasn't been suggested yet (that I've seen): skip everything...

  

Hasn't been suggested because it won't work.


put the length of url "file:myfile.txt" into tEnd
open file "myfile.txt" for text update
seek to tEnd in file "myfile.txt"

>From here just back up a "reasonable" number of characters... say 200. Find
the last CR character, nuke everything else and close the file. Didn't find
one? Try backing up another 200, etc. It'll be a whole lot faster.

  

"... nuke everything else " ?? How ?
You've opened the file for update, so anything that you write to the 
file *overwrites* any existing characters at the same position(s), and 
leaves everything following that unchanged. So there is no way to 
shorten a file if opened in update mode.


I *think* the best you can do is

1. find file length (best to use the detailed files)
2. open for read, and read some chunk at the end (200, 1000, whatever )
  (open for read is more efficient than open for update)
3. calculate the number of bytes to be deleted off the end of the file
4. close the file
5. open the file for append
6. write empty to the file at the appropriate point

Edge case to concern yourself with would be if your file happens to end with
a CR and maybe you want to ignore those cases. But that should be easy
enough to solve.

  
Easiest (and I think best) way to resolve that corner case is to 
preserve the trailing CR if it's there to begin with, and not leave one 
if it wasn't there to start with. This happens to involve no extra work 
at all, hence it's the easiest one :-)


I decided it was time to email some (reasonably tested) revTalk rather 
than just ideas 


(NB assumes that the defaultfolder has already been set to the 
appropriate place)

on deletelastline pFile
   constant K = 1000
   put the detailed files into t
   filter t with pFile & ",*"
   put item 2 of t into tFileLength
   
   open file pFile for read

   seek to tFileLength-K in  file pFile
   read from file pFile until end
   close file pFile
   
   put the number of chars in the last line of it into tNum
   
   open file pFile for append

   write empty to file pFile at (tFileLength-tNum-3)
   close file pFile
   
   end deletelastline

-- Alex.
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Re: Reading/Deleting Last Line Of File

[Brian Yennie]

It's more a product of the syntax than anything. url "file:myfile.txt" is a 
container, and the length operates on it as such. URL containers are pretty 
darn handy in general, but this is one potential gotcha.
You can use the detailed files to get information about a file, including its 
size in bytes. So actually your suggestion should work fine, just needs a 
little tweaking. It would just be more like:

put filelength("myfile.txt") into tEnd
open file "myfile.txt" for text update
seek to tEnd in file "myfile.txt"

function filelength tFile
// use the detailed files to get the file size
end filelength

No need to get all flustered =)

- Brian

> 
> 
> On Feb 10, 2010, at 11:26 PM, Brian Yennie wrote:
> 
>> Jeff,
>> 
>> This line:
>> 
>>> put the length of url "file:myfile.txt" into tEnd
>> 
>> Loads the entire file into memory in order to get its length.
> 
> 
> 
> That's a joke, right? :-(
> 
> A freakin' OS call could get that just by touching an I-Node. Please, God, 
> tell me Rev was smart enough to do it the right way?
> 
> Jeff M.___
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Re: Reading/Deleting Last Line Of File

[Jeffrey Massung]

On Feb 10, 2010, at 11:26 PM, Brian Yennie wrote:

> Jeff,
> 
> This line:
> 
>> put the length of url "file:myfile.txt" into tEnd
> 
> Loads the entire file into memory in order to get its length.



That's a joke, right? :-(

A freakin' OS call could get that just by touching an I-Node. Please, God, tell 
me Rev was smart enough to do it the right way?

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Re: Reading/Deleting Last Line Of File

[Brian Yennie]

Jeff,

This line:

> put the length of url "file:myfile.txt" into tEnd

Loads the entire file into memory in order to get its length.

- Brian
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Re: Reading/Deleting Last Line Of File

[Jeff Massung]

Warren,

I've read through most of these suggestions, but I'm surprised that the
obvious hasn't been suggested yet (that I've seen): skip everything...

put the length of url "file:myfile.txt" into tEnd
open file "myfile.txt" for text update
seek to tEnd in file "myfile.txt"

>From here just back up a "reasonable" number of characters... say 200. Find
the last CR character, nuke everything else and close the file. Didn't find
one? Try backing up another 200, etc. It'll be a whole lot faster.

Edge case to concern yourself with would be if your file happens to end with
a CR and maybe you want to ignore those cases. But that should be easy
enough to solve.

HTH,

Jeff M.

On Wed, Feb 10, 2010 at 7:06 AM, Warren Kuhl  wrote:

> Thanks for all the suggestions.  I will look into trying some of them
> to see what works best for me.  The file currently contains
> 105,750,304 records.
>
> I will report back my findings.
>
> Thanks,
> Warren
>
>
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Re: Reading/Deleting Last Line Of File

[Kay C Lan]

You mean you'll report back when you only have 105,750,303 records ;-)

On Wed, Feb 10, 2010 at 9:06 PM, Warren Kuhl  wrote:

> Thanks for all the suggestions.  I will look into trying some of them
> to see what works best for me.  The file currently contains
> 105,750,304 records.
>
> I will report back my findings.
>
> Thanks,
> Warren
>
>
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Re: Reading/Deleting Last Line Of File

[Warren Kuhl]

Thanks for all the suggestions.  I will look into trying some of them
to see what works best for me.  The file currently contains
105,750,304 records.

I will report back my findings.

Thanks,
Warren

On Tue, Feb 9, 2010 at 11:58 PM, J. Landman Gay
 wrote:
> Jim Bufalini wrote:
>>
>> Jacque wrote:
>>
>>> Worth a test anyway. I've used it on files that were several megs in
>>> size without a problem, but they weren't super huge, just kinda big.
>>
>> Hi Jacque, I made the suggestion I did because at 100 Million records plus
>
> 100 million? Yes, well...I think I read the zeros wrong.
>
> --
> Jacqueline Landman Gay         |     jac...@hyperactivesw.com
> HyperActive Software           |     http://www.hyperactivesw.com
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Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

Jim Bufalini wrote:

Jacque wrote:


Worth a test anyway. I've used it on files that were several megs in
size without a problem, but they weren't super huge, just kinda big.


Hi Jacque, I made the suggestion I did because at 100 Million records plus


100 million? Yes, well...I think I read the zeros wrong.

--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Hey Richard,

Well now we presumably have three different ways to do it. Sounds like a job
for Mr. Gaskin to test which is the fastest. I'd offer to test but am plumb
out of 100 MM record data files. LOL.

Aloha from Hawaii,

Jim Bufalini

> -Original Message-
> From: use-revolution-boun...@lists.runrev.com [mailto:use-revolution-
> boun...@lists.runrev.com] On Behalf Of Richard Gaskin
> Sent: Tuesday, February 09, 2010 2:10 PM
> To: How to use Revolution
> Subject: Re: Reading/Deleting Last Line Of File
> 
> See why you shouldn't trust off-the-cuff code?
> 
> Where I wrote:
> 
> open file tFile for append
> 
> ...I meant to write:
> 
> open file tFile for update
> 
> --
>   Richard Gaskin
>   Fourth World
>   Rev training and consulting: http://www.fourthworld.com
>   Webzine for Rev developers: http://www.revjournal.com
>   revJournal blog: http://revjournal.com/blog.irv
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Jacque wrote:

> Worth a test anyway. I've used it on files that were several megs in
> size without a problem, but they weren't super huge, just kinda big.

Hi Jacque, I made the suggestion I did because at 100 Million records plus,
that's an awfully big read into memory just to delete the last line, so I
figured it's better to work with the file on disk. I just remembered the
params for open file wrong and Alex caught it.

Aloha from Hawaii,

Jim Bufalini

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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Alex,

It does work, just not "exactly" how I said to do it. ;-) You have to open
the file for "append*, instead of *write* and then write at a byte offset.
So:

open file "" for append
write "This is the EOF" to file "" at 
close file ""

I just tested on an approximately 10kb text file and if  is 5000, the
"This is the EOF" is written starting at byte 5000 and the file is
terminated at the end of that string and the file data before byte 5000 is
retained in the file and the file changes to an approximately a 5KB file, so
the EOF is moved and the file is truncated.

I really shouldn't dash these answers off from memory. ;-)

Aloha from Hawaii,

Jim Bufalini



Alex Tweedly wrote:

> I don't think this will work, Jim.  If you open a file for write, then
> it erases the entire content of the file first
> 
> as the docs say (emphasis added)...
> > The file to write to must be opened first with the open file command,
> > and the mode the file was opened in must be write, append, or update.
> > If the file is not open or is open read-only, the result function is
> > set to "File is not open for write.".
> >
> > If the file was opened in write mode, the write to file command
> > completely *replaces the file contents from the start.* For example,
> > if the file originally contains "ABC", and you write "1" to it, after
> > the write to file command is executed the file contains "1".
> And as you say, open for update does not allow you to truncate the
> file.
> 
> The standard C library has a way to do it, and there is a Unix shell
> command - but not afaict a Windows shell utility to do it.
> 
> -- Alex
> 
> 
> 
> Jim Bufalini wrote:
> > Warren,
> >
> >
> >> I have a large text file (100,000,000+ records).  I need to write a
> >> utility that removes the last record of the file.  Is there anyway
> to
> >> just remove the last record without reading through the complete
> file
> >> with RunRev?
> >>
> >
> > This is totally untested but "logically" you could:
> >
> > 1. First get the size of the file on disk and approximate a character
> offset
> > that should be only a few lines from the end of the file.
> >
> > 2. Then Open the file for read and read from file starting at the
> char
> > offset you calculated to EOF into a var and delete the last line of
> the var.
> >
> >
> > 3. Then Close the file and Open it again for write.
> >
> > 4. Now write the var to file at the same starting offset you read
> from.
> >
> > The reason you don't use open for update is that you want to open the
> file
> > for write, so that whatever you write to the file replaces everything
> from
> > the offset to the EOF. Update will replace chars and leave the last
> record
> > at the end.
> >
> > What I don't know here is if internally the read/write from file at
> start
> > still has to go through the entire file to calculate the starting
> offset, so
> > I don't know how long this will take on a file of your size. But, at
> least,
> > doing it this way won't require reading the whole file into memory
> and
> > should result in the last line being deleted from the file in the
> quickest
> > way available.
> >
> > Aloha from Hawaii,
> >
> > Jim Bufalini
> >
> >
> >
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Re: Reading/Deleting Last Line Of File

[Richard Gaskin]

See why you shouldn't trust off-the-cuff code?

Where I wrote:

   open file tFile for append

...I meant to write:

   open file tFile for update

--
 Richard Gaskin
 Fourth World
 Rev training and consulting: http://www.fourthworld.com
 Webzine for Rev developers: http://www.revjournal.com
 revJournal blog: http://revjournal.com/blog.irv
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Re: Reading/Deleting Last Line Of File

[Richard Gaskin]

Would the seek command help?

Presumably it would move the file pointer to the specified point without 
having to load the entire file into RAM.


If you knew your lines were shorter than a given length, say 1000 chars, 
you could so something like:


  open file tFile for append
  seek relative -1000 in file tFile
  repeat
 read from file tFile until cr
 if it is not empty then
put it after tBuffer
 else
delete last line of tBuffer
write tBuffer to file tFile
 end if
  end repeat
  close file tFile

This is off the top of my head to test before using on real data. ;)

--
 Richard Gaskin
 Fourth World
 Rev training and consulting: http://www.fourthworld.com
 Webzine for Rev developers: http://www.revjournal.com
 revJournal blog: http://revjournal.com/blog.irv
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Re: Reading/Deleting Last Line Of File

[J. Landman Gay]

Piggy-backing here:

I have a large text file (100,000,000+ records).  I need to write a
utility that removes the last record of the file.  Is there anyway to
just remove the last record without reading through the complete file
with RunRev?


It sounds like a big file, but Rev is pretty good about managing memory 
and swapping to disk. I'd be tempted to try this and see how it works:


  delete line -1 of url ("file:mytext.txt")

Worth a test anyway. I've used it on files that were several megs in 
size without a problem, but they weren't super huge, just kinda big.


--
Jacqueline Landman Gay | jac...@hyperactivesw.com
HyperActive Software   | http://www.hyperactivesw.com
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Re: Reading/Deleting Last Line Of File

[Alex Tweedly]

I don't think this will work, Jim.  If you open a file for write, then 
it erases the entire content of the file first


as the docs say (emphasis added)...
The file to write to must be opened first with the open file command, 
and the mode the file was opened in must be write, append, or update. 
If the file is not open or is open read-only, the result function is 
set to "File is not open for write.".


If the file was opened in write mode, the write to file command 
completely *replaces the file contents from the start.* For example, 
if the file originally contains "ABC", and you write "1" to it, after 
the write to file command is executed the file contains "1".

And as you say, open for update does not allow you to truncate the file.

The standard C library has a way to do it, and there is a Unix shell 
command - but not afaict a Windows shell utility to do it.


-- Alex



Jim Bufalini wrote:

Warren,

  

I have a large text file (100,000,000+ records).  I need to write a
utility that removes the last record of the file.  Is there anyway to
just remove the last record without reading through the complete file
with RunRev?



This is totally untested but "logically" you could:
 
1. First get the size of the file on disk and approximate a character offset
that should be only a few lines from the end of the file. 


2. Then Open the file for read and read from file starting at the char
offset you calculated to EOF into a var and delete the last line of the var.


3. Then Close the file and Open it again for write.

4. Now write the var to file at the same starting offset you read from.

The reason you don't use open for update is that you want to open the file
for write, so that whatever you write to the file replaces everything from
the offset to the EOF. Update will replace chars and leave the last record
at the end.

What I don't know here is if internally the read/write from file at start
still has to go through the entire file to calculate the starting offset, so
I don't know how long this will take on a file of your size. But, at least,
doing it this way won't require reading the whole file into memory and
should result in the last line being deleted from the file in the quickest
way available.

Aloha from Hawaii,

Jim Bufalini



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Re: Reading/Deleting Last Line Of File

[Warren Kuhl]

Jim,

Thanks for the explanationI will give this a try.

Thanks!
Warren

On Tue, Feb 9, 2010 at 9:08 AM, Jim Bufalini  wrote:
> Warren,
>
>> I will give this a try.  I was trying to go down the same path by
>> getting the number of records of the file...then trying to determine
>> if I could just read the last line and removing the EOF.  Is there
>> anyway to specifiy a READ to a certain line of a file without reading
>> through the whole file?
>
> The read from file command allows you start at a character offset and then
> read to a string, or for a number of bytes/chars/words/items (see
> dictionary) or to EOF. The write command will replace from the starting
> character offset and move the EOF for you. So, in your case, I don't think
> you have to be exact in your read. Just read in close to the end to EOF and
> write from the same offset and let the write command handle the EOF for you.
>
> Aloha from Hawaii,
>
> Jim Bufalini
>
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Warren,

> I will give this a try.  I was trying to go down the same path by
> getting the number of records of the file...then trying to determine
> if I could just read the last line and removing the EOF.  Is there
> anyway to specifiy a READ to a certain line of a file without reading
> through the whole file?

The read from file command allows you start at a character offset and then
read to a string, or for a number of bytes/chars/words/items (see
dictionary) or to EOF. The write command will replace from the starting
character offset and move the EOF for you. So, in your case, I don't think
you have to be exact in your read. Just read in close to the end to EOF and
write from the same offset and let the write command handle the EOF for you.

Aloha from Hawaii,

Jim Bufalini

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Re: Reading/Deleting Last Line Of File

[Warren Kuhl]

Jim,

I will give this a try.  I was trying to go down the same path by
getting the number of records of the file...then trying to determine
if I could just read the last line and removing the EOF.  Is there
anyway to specifiy a READ to a certain line of a file without reading
through the whole file?

Warren

On Tue, Feb 9, 2010 at 8:34 AM, Jim Bufalini  wrote:
> Warren,
>
>> I have a large text file (100,000,000+ records).  I need to write a
>> utility that removes the last record of the file.  Is there anyway to
>> just remove the last record without reading through the complete file
>> with RunRev?
>
> This is totally untested but "logically" you could:
>
> 1. First get the size of the file on disk and approximate a character offset
> that should be only a few lines from the end of the file.
>
> 2. Then Open the file for read and read from file starting at the char
> offset you calculated to EOF into a var and delete the last line of the var.
>
>
> 3. Then Close the file and Open it again for write.
>
> 4. Now write the var to file at the same starting offset you read from.
>
> The reason you don't use open for update is that you want to open the file
> for write, so that whatever you write to the file replaces everything from
> the offset to the EOF. Update will replace chars and leave the last record
> at the end.
>
> What I don't know here is if internally the read/write from file at start
> still has to go through the entire file to calculate the starting offset, so
> I don't know how long this will take on a file of your size. But, at least,
> doing it this way won't require reading the whole file into memory and
> should result in the last line being deleted from the file in the quickest
> way available.
>
> Aloha from Hawaii,
>
> Jim Bufalini
>
>
>
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RE: Reading/Deleting Last Line Of File

[Jim Bufalini]

Warren,

> I have a large text file (100,000,000+ records).  I need to write a
> utility that removes the last record of the file.  Is there anyway to
> just remove the last record without reading through the complete file
> with RunRev?

This is totally untested but "logically" you could:
 
1. First get the size of the file on disk and approximate a character offset
that should be only a few lines from the end of the file. 

2. Then Open the file for read and read from file starting at the char
offset you calculated to EOF into a var and delete the last line of the var.


3. Then Close the file and Open it again for write.

4. Now write the var to file at the same starting offset you read from.

The reason you don't use open for update is that you want to open the file
for write, so that whatever you write to the file replaces everything from
the offset to the EOF. Update will replace chars and leave the last record
at the end.

What I don't know here is if internally the read/write from file at start
still has to go through the entire file to calculate the starting offset, so
I don't know how long this will take on a file of your size. But, at least,
doing it this way won't require reading the whole file into memory and
should result in the last line being deleted from the file in the quickest
way available.

Aloha from Hawaii,

Jim Bufalini



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Re: Reading/Deleting Last Line Of File

[Till Bandi]

doesn't that work?:

delete line -1 of file myTextFile

Till
Am 09.02.2010 um 14:52 schrieb Warren Kuhl:

> I have a large text file (100,000,000+ records).  I need to write a
> utility that removes the last record of the file.  Is there anyway to
> just remove the last record without reading through the complete file
> with RunRev?
> 
> Thanks,
> Warren
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