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From: Can Baran on 6 Apr 2010 15:13
Hey guys,
So, I want to overwrite some lines in a txt file via fopen.
so
fid = fopen(myfile, 'w+');
cur_line = fgetl(di);
while ischar(cur_line)
%if changeable(cur_line)
%change it
end
fclose(fid)
however when i open with w+ (to overwrite), fgetl always returns -1. Why?
I read the help it says:
the line contains only the end-of-file marker. In this case, tline is the numeric value -1.
How can I go line by line after opening a file with w+
or
how can I overwrite a txt file if I open it with r+
Can
From: Jan Simon on 6 Apr 2010 15:43
Dear Can!

> So, I want to overwrite some lines in a txt file via fopen.

> fid = fopen(myfile, 'w+');
> cur_line = fgetl(di);

> however when i open with w+ (to overwrite), fgetl always returns -1. Why?

help fopen

There you find:
'w+' truncate or create for read and write
So after FOPEN('w+') the file is empty. I assume you want to FOPEN('r+').

I think that "fgetl(di)" should be "fgetl(fid)".

Good luck, Jan
From: bcb on 6 Apr 2010 15:45
On Tue, 06 Apr 2010 19:13:05 +0000, Can Baran wrote:

> Hey guys,
> So, I want to overwrite some lines in a txt file via fopen. so
> fid = fopen(myfile, 'w+');
> cur_line = fgetl(di);
> while ischar(cur_line)
> %if changeable(cur_line)
> %change it
> end
> fclose(fid)
> however when i open with w+ (to overwrite), fgetl always returns -1.
> Why? I read the help it says:
> the line contains only the end-of-file marker. In this case, tline is
> the numeric value -1. How can I go line by line after opening a file
> with w+ or
> how can I overwrite a txt file if I open it with r+ Can

Did you read the help for fopen?

'w+' Open or create new file for reading and writing. **Discard
existing contents, if any.** (emphasis mine)

fgetl returns -1 because there is no data in the file anymore...

pidgen code

open old file
open new file
while there's data in the old file
read old file
change data as appropriate
write new file
when done, rename new file to old file

Bruce
From: Can Baran on 6 Apr 2010 16:58
bcb <bbowler(a)bigelow.org> wrote in message <821h9tFsm1U1(a)mid.individual.net>...
> On Tue, 06 Apr 2010 19:13:05 +0000, Can Baran wrote:
>
> > Hey guys,
> > So, I want to overwrite some lines in a txt file via fopen. so
> > fid = fopen(myfile, 'w+');
> > cur_line = fgetl(di);
> > while ischar(cur_line)
> > %if changeable(cur_line)
> > %change it
> > end
> > fclose(fid)
> > however when i open with w+ (to overwrite), fgetl always returns -1.
> > Why? I read the help it says:
> > the line contains only the end-of-file marker. In this case, tline is
> > the numeric value -1. How can I go line by line after opening a file
> > with w+ or
> > how can I overwrite a txt file if I open it with r+ Can
>
> Did you read the help for fopen?
>
> 'w+' Open or create new file for reading and writing. **Discard
> existing contents, if any.** (emphasis mine)
>
> fgetl returns -1 because there is no data in the file anymore...
>
> pidgen code
>
> open old file
> open new file
> while there's data in the old file
> read old file
> change data as appropriate
> write new file
> when done, rename new file to old file
>
> Bruce

Bruce,
you see that you write each line of the old file to the new file
what i want to do is i dont want to create a new file, i want to parse the old file until i find the thing i am interested in then i want to change that in the **old file** and break the while loop and output the old file
and yes i did read the help for fopen, but what does it mean that it discards existing contents? I thought that enables you to overwrite it.
Currently i am implementing my code the way you recommend it in your pseudo-code, but there should be a better way of doing this (i.e. without creating a new file, changing the old file)
From: TideMan on 6 Apr 2010 17:10
On Apr 7, 8:58 am, "Can Baran" <can_bara...(a)yahoo.com> wrote:
> bcb <bbow...(a)bigelow.org> wrote in message <821h9tFsm...(a)mid.individual.net>...
> > On Tue, 06 Apr 2010 19:13:05 +0000, Can Baran wrote:
>
> > > Hey guys,
> > > So, I want to overwrite some lines in a txt file via fopen. so
> > > fid = fopen(myfile, 'w+');
> > > cur_line = fgetl(di);
> > > while ischar(cur_line)
> > > %if changeable(cur_line)
> > > %change it
> > > end
> > > fclose(fid)
> > > however when i open with w+ (to overwrite), fgetl always returns -1.
> > > Why? I read the help it says:
> > > the line contains only the end-of-file marker. In this case, tline is
> > > the numeric value -1. How can I go line by line after opening a file
> > > with w+ or
> > > how can I overwrite a txt file if I open it with r+ Can
>
> > Did you read the help for fopen?
>
> > 'w+'       Open or create new file for reading and writing. **Discard        
> >            existing contents, if any.**  (emphasis mine)
>
> > fgetl returns -1 because there is no data in the file anymore...
>
> > pidgen code
>
> >    open old file
> >    open new file
> >    while there's data in the old file
> >            read old file
> >            change data as appropriate
> >            write new file
> >    when done, rename new file to old file
>
> > Bruce
>
> Bruce,
>  you see that you write each line of the old file to the new file
> what i want to do is i dont want to create a new file, i want to parse the old file until i find the thing i am interested in then i want to change that in the **old file** and break the while loop and output the old file
> and yes i did read the help for fopen, but what does it mean that it discards existing contents? I thought that enables you to overwrite it.
> Currently i am implementing my code the way you recommend it in your pseudo-code, but there should be a better way of doing this (i.e. without creating a new file, changing the old file)

Well, one way is to read the entire file in using:
fid=fopen(myfile,'rt');
a=fscanf(fid,'%c');
fclose(fid);
fid=fopen(myfile,'wt');
Now, you use strrep to replace strings in a as required, then:
fprintf(fid,'%c',a);
fclose(fid);

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