Tcl - In-place file record modification

Author

In-place file record modification

Andy

2004-12-27, 3:58 pm

Hi,

I would like to know if one can do in-place file editing in Tcl.
Example:
I have record such as
0078 100,99,10,11,11,0,0

to be replaced by
0078 100,99,10,11,11,7,9

Can one replace this record in this file without creating a new
file ?

Thanks in advance,
Anand

Arjen Markus

2004-12-27, 3:58 pm

Andy wrote:
>
> Hi,
>
> I would like to know if one can do in-place file editing in Tcl.
> Example:
> I have record such as
> 0078 100,99,10,11,11,0,0
>
> to be replaced by
> 0078 100,99,10,11,11,7,9
>
> Can one replace this record in this file without creating a new
> file ?
>
> Thanks in advance,
> Anand

Yes, you can. See the script below:

set outfile [open "updfile.txt" "w"]
puts $outfile "First write some text ...."
close $outfile

set inout [open "updfile.txt" "r+"]

puts $inout "xxxxx"

close $inout

Note:
- "r+" will do the trick
- puts will add an extra newline, unless you use the -nonewline option
- you can not _insert_ characters

Regards,

Arjen

Victor Wagner

2004-12-27, 3:58 pm

Andy <kaleanand@gmail.com> wrote:
: I would like to know if one can do in-place file editing in
Tcl.
: Example:
: I have record such as
: 0078 100,99,10,11,11,0,0
: to be replaced by
: 0078 100,99,10,11,11,7,9
: Can one replace this record in this file without creating a
new
: file ?

You can do it same way as text editors do: load entire file into memory,
modify there and save back. It is quite easy in Tcl

set f [open filename]
set content [read $f]
# insert code to modify content variable here
set f [open filename w]
puts -nonewline $content
close $f.

There is also possiblity to do random access to the file using s

command. But it is scarcely applicable to your case, because it would
quite hard work if line length changes during editing.

--

Andy

2004-12-27, 3:58 pm

Hi,

Thanks guys for the tips ..

I use the tclX package, I can do something like this

set line [bsearch $key $fp]
# Modify the record returned by this
puts -nonewline $fp $line # Assuming the file is opened in overwrite
mode.

Any comments

Michael Schlenker

2004-12-27, 3:58 pm

Andy schrieb:
> Hi,
>
> Thanks guys for the tips ..
>
> I use the tclX package, I can do something like this
>
> set line [bsearch $key $fp]
> # Modify the record returned by this
> puts -nonewline $fp $line # Assuming the file is opened in overwrite
> mode.
>
> Any comments
>
The only way it is doable is for fixed size records. If you have those,
open the file with open, search the record to modify (or calculate its
position from your information about the format), use s

to get there,
write your fixed sized record over the one in question and close the
file again. But if you need this kind of functionality it is usually a
better idea to use a real database like Metakit or SQLite for storing
the data.

Michael

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