Problem rewinding the __DATA__ filehandle

[Shaji Kalidasan]

Dear Perlers,

I am trying to print the matching lines using __DATA__ filehandle and for the 
very first time it prints the desired lines, but as soon as I rewind it using 
seek, it is printing lines from the very beginning which is not the desired 
result (as it prints from the start). I want to rewind only the __DATA__ part.

Please help to print the matching lines using approach 2 and 3 as well.

[code]
use strict;
use warnings;

#Approach 1
#Printing lines 2 through 5 using range operator
while (DATA) {
print if 2 .. 5;
}

print Info: The position of DATA file handle is , tell DATA, \n;
seek DATA, 0, 0;
print Info: The position of DATA file handle is , tell DATA, \n;

#Approach 2
#Printing lines 2 through 5 using counter.
my $counter = 1;
while (DATA) {
print if $counter = 2  $counter = 5;
$counter++;
}

print Info: The position of DATA file handle is , tell DATA, \n;
seek DATA, 0, 0;
print Info: The position of DATA file handle is , tell DATA, \n;

#Printing lines 2 through 5 using $.
while (DATA) {
print if $. = 2  $. = 5;
}

__DATA__
India
Srilanka
USA
Nepal
France
UK
Australia
New Zealand

[/code]

[output]
Srilanka
USA
Nepal
France
Info: The position of DATA file handle is 777
Info: The position of DATA file handle is 0
use warnings;

#Approach 1
#Printing lines 2 through 5 using range operator
Info: The position of DATA file handle is 777
Info: The position of DATA file handle is 0
[/output]


Thank you.
 
best,
Shaji 
---
Your talent is God's gift to you. What you do with it is your gift back to God.
---

Re: Problem rewinding the __DATA__ filehandle

[Jim Gibson]

On Oct 25, 2013, at 4:46 PM, Shaji Kalidasan wrote:

 Dear Perlers,
 
 I am trying to print the matching lines using __DATA__ filehandle and for the 
 very first time it prints the desired lines, but as soon as I rewind it using 
 seek, it is printing lines from the very beginning which is not the desired 
 result (as it prints from the start). I want to rewind only the __DATA__ part.

As you have discovered, the special file handle DATA is opened to the source 
file for your program. When the compiler encounters either of the following 
lines in your source file:

__END__
__DATA__


it stops reading the source file and leaves the file handle open at that point. 
The first time you read the DATA file handle, you get the line after the 
'__DATA__' line. But when you rewind the file handle, it is positioned at the 
beginning of the file, and you get to read your program.

You have two choices (at least):

1. Save the position of the file handle when you enter the program:

  my $data_pos = tell(DATA);

Then reposition the file to this point instead of at the beginning of the file:

  seek( DATA, $data_pos, 0 );


2. After rewinding the file to the beginning with seek(DATA,0,0), read the file 
until you encounter the '__DATA__' line. Then start reading the data lines.



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Re: Problem rewinding the __DATA__ filehandle

[Paul Johnson]

On Fri, Oct 25, 2013 at 05:14:23PM -0700, Jim Gibson wrote:
 
 On Oct 25, 2013, at 4:46 PM, Shaji Kalidasan wrote:
 
  Dear Perlers,
  
  I am trying to print the matching lines using __DATA__ filehandle and for 
  the very first time it prints the desired lines, but as soon as I rewind it 
  using seek, it is printing lines from the very beginning which is not the 
  desired result (as it prints from the start). I want to rewind only the 
  __DATA__ part.
 
 As you have discovered, the special file handle DATA is opened to the source 
 file for your program. When the compiler encounters either of the following 
 lines in your source file:
 
 __END__
 __DATA__
 
 
 it stops reading the source file and leaves the file handle open at that 
 point. The first time you read the DATA file handle, you get the line after 
 the '__DATA__' line. But when you rewind the file handle, it is positioned at 
 the beginning of the file, and you get to read your program.
 
 You have two choices (at least):
 
 1. Save the position of the file handle when you enter the program:
 
   my $data_pos = tell(DATA);
 
 Then reposition the file to this point instead of at the beginning of the 
 file:
 
   seek( DATA, $data_pos, 0 );
 
 
 2. After rewinding the file to the beginning with seek(DATA,0,0), read the 
 file until you encounter the '__DATA__' line. Then start reading the data 
 lines.

The first choice is the correct one.

Ideally, you should use the SEEK_SET constant from Fcntl.  perldoc Fcntl
for details.

And for your third approach, you need C $. = 0; 

-- 
Paul Johnson - p...@pjcj.net
http://www.pjcj.net

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Re: Problem rewinding the __DATA__ filehandle

[Shaji Kalidasan]

Dear Jim,

Awesome, thanks a bunch for your detailed explanation. It works like a champ.
 
best,
Shaji 
---
Your talent is God's gift to you. What you do with it is your gift back to God.
---



On Saturday, 26 October 2013 5:44 AM, Jim Gibson jimsgib...@gmail.com wrote:
 

On Oct 25, 2013, at 4:46 PM, Shaji Kalidasan wrote:

 Dear Perlers,
 
 I am trying to print the matching lines using __DATA__ filehandle and for the 
 very first time it prints the desired lines, but as soon as I rewind it using 
 seek, it is printing lines from the very beginning which is not the desired 
 result (as it prints from the start). I want to rewind only the __DATA__ part.

As you have discovered, the special file handle DATA is opened to the source 
file for your program. When the compiler encounters either of the following 
lines in your source file:

__END__

__DATA__


it stops reading the source file and leaves the file handle open at that point. 
The first time you read the DATA file handle, you get the line after the 
'__DATA__' line. But when you rewind the file handle, it is positioned at the 
beginning of the file, and you get to read your program.

You have two choices (at least):

1. Save the position of the file handle when you enter the program:

  my $data_pos = tell(DATA);

Then reposition the file to this point instead of at the beginning of the file:

  seek( DATA, $data_pos, 0 );


2. After rewinding the file to the beginning with seek(DATA,0,0), read the file 
until you encounter the '__DATA__' line. Then start reading the data lines.



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Re: Problem rewinding the __DATA__ filehandle

[John W. Krahn]

Jim Gibson wrote:


On Oct 25, 2013, at 4:46 PM, Shaji Kalidasan wrote:


Dear Perlers,

I am trying to print the matching lines using __DATA__ filehandle and

for the very first time it prints the desired lines, but as soon as I
rewind it using seek, it is printing lines from the very beginning
which is not the desired result (as it prints from the start). I want
to rewind only the __DATA__ part.

As you have discovered, the special file handle DATA is opened to the
source file for your program. When the compiler encounters either of
the following lines in your source file:

__END__
__DATA__


it stops reading the source file and leaves the file handle open at
that point. The first time you read theDATA  file handle, you get
the line after the '__DATA__' line. But when you rewind the file
handle, it is positioned at the beginning of the file, and you get to
read your program.

You have two choices (at least):

1. Save the position of the file handle when you enter the program:

   my $data_pos = tell(DATA);


DATA is the same as readline( DATA ) which will not work with tell().

It should be just:

   my $data_pos = tell DATA;



John
--
Any intelligent fool can make things bigger and
more complex... It takes a touch of genius -
and a lot of courage to move in the opposite
direction.   -- Albert Einstein

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Re: Problem rewinding the __DATA__ filehandle

[Shaji Kalidasan]

Dear Paul,

Many thanks for your explanation on Approach 3

Here is the working code

Courtesy : 

1) Jim Gibson (For Approach 1 and 2)
2) Paul Johnson (For Approach 3)
3) John W Krahn (I figured it out myself, thanks for emphasizing the fact to 
greater audience)

[code]
use strict;
use warnings;

my $data_position = tell DATA;
#Approach 1
#Printing lines 2 through 5 using range operator
while (DATA) {
print if 2 .. 5;
}

print Info: The position of DATA file handle is , tell DATA, \n;
seek DATA, $data_position, 0;
print Info: The position of DATA file handle is , tell DATA, \n;

#Approach 2
#Printing lines 2 through 5 using counter.
my $counter = 1;
while (DATA) {
print if $counter = 2  $counter = 5;
$counter++;
}

print Info: The position of DATA file handle is , tell DATA, \n;
seek DATA, $data_position, 0;
print Info: The position of DATA file handle is , tell DATA, \n;

#Approach 3
#Printing lines 2 through 5 using $.
$. = 0;
while (DATA) {
print if $. = 2  $. = 5;
}

__DATA__
India
Srilanka
USA
Nepal
France
UK
Australia
New Zealand
[/code]

[output]
Srilanka
USA
Nepal
France
Info: The position of DATA file handle is 855
Info: The position of DATA file handle is 792
Srilanka
USA
Nepal
France
Info: The position of DATA file handle is 855
Info: The position of DATA file handle is 792
Srilanka
USA
Nepal
France
[/output]
 
best,
Shaji 
---
Your talent is God's gift to you. What you do with it is your gift back to God.
---



On Saturday, 26 October 2013 8:56 AM, John W. Krahn jwkr...@shaw.ca wrote:
 
Jim Gibson wrote:

 On Oct 25, 2013, at 4:46 PM, Shaji Kalidasan wrote:

 Dear Perlers,

 I am trying to print the matching lines using __DATA__ filehandle and
 for the very first time it prints the desired lines, but as soon as I
 rewind it using seek, it is printing lines from the very beginning
 which is not the desired result (as it prints from the start). I want
 to rewind only the __DATA__ part.

 As you have discovered, the
 special file handle DATA is opened to the

 source file for your program. When the compiler encounters either of
 the following lines in your source file:

 __END__
 __DATA__


 it stops reading the source file and leaves the file handle open at
 that point. The first time you read theDATA  file handle, you get
 the line after the '__DATA__' line. But when you rewind the file
 handle, it is positioned at the beginning of the file, and you get to
 read your program.

 You have two choices (at least):

 1. Save the position of the file handle when you enter the program:

    my $data_pos = tell(DATA);

DATA is the same as readline( DATA ) which will not work with tell().

It should be just:

    my $data_pos = tell DATA;



John
-- 
Any intelligent fool can make things bigger and
more complex... It takes a touch of genius -
and a lot of courage to move in the opposite
direction.                   -- Albert Einstein


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