hari has asked for the wisdom of the Perl Monks concerning the following question:

Hi all,
$currentline = <AFILE>;
if($currentline =~ /ENCRYPTED/)
{
   #here i like to point to previous line
   #so that if i call<AFILE>again it returns the same line.
   #without using FileHandle
   #write(*AFILE);
}
else
{
  parse($currentLine);
}

[download]
anybody know how to do it.
Thanks
Harish

Edited 2003-03-05 by mirod:added <code> tags

Replies are listed 'Best First'.
Re: point to previous line
by tachyon (Chancellor) on Mar 05, 2003 at 09:25 UTC

    tell and seek will let you do it but I don't see why you would need to. 

    my $pos = tell FH;
$line = <FH>;
seek FH, $pos, 0;
# we will now read the same line if we do $line = <FH>;

    [download]

    cheers

    tachyon

    s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

[reply]
[d/l]
      You can also read the file into an array, then any navigation on the file is easy using array index:
      my @file = <AFILE>;    # reading all file to an array
for (@file) {
  ...
}

      [download]


      Hotshot
[reply]
[d/l]
      hi , tell and seek, gives and sets to the same position, but the line retrived after that is not complete.
      What do you think what may be the problem. most of the time it is 4 character afront but not consistently.
      Does seek sub neglects spaces?
      Hari
[reply]
Re: point to previous line
by rir (Vicar) on Mar 05, 2003 at 14:09 UTC
    You can also use a queue.

    All off-hand code. 

    my @q[0..1] = ( "\n" x 2);  # other initialization may be apropos

while (1) {
   @q[0..1] = ($q[1], scalar( <INPUT>));
   last if not defined $q[1];
   # previous line is $q[0]
   # currentline is $q[1]
   do_something(\@q);
}
# current line, the last line is now in $q[0]

    [download]

    Something like this may be used to empty the queue inside the loop. Replace the line with "last" with this: 

    # handle last line in loop
    if ( not defined $q[1] ) {
         $q[1] = $q[0];
         $q[0] = "\n";
          do_something( \@q);
          last;
     }

    [download]
[reply]
[d/l]
[select]
Re: point to previous line
by Limbic~Region (Chancellor) on Mar 05, 2003 at 14:12 UTC
    hari,
    I am not exactly sure I understand what you are trying to do. You say that you want to point to the previous line, so that if you read from the file handle, it returns the same line. Returning the same line is not the same as retrieving the previous line.

    Since I do not completely understand - let me offer some possible alternatives:

  • You could use the "redo" command in a while loop if you want the same line and not the previous line
  • You could read each line in and store the previous line in a buffer
  • You could mark each position using tell and seek back as has been suggested
  • You could store the file in an array as has been suggested if memory is not an issue
  • You could use Tie::File (which treats a file as an array) if memory is an issue, but speed is not

    To help you more than that - you need to explain more clearly what you are trying to do by giving a sample file and showing what you want to do at each point in the file.

    Hope this helps - cheers - L~R

    Update: While I was working on posting this, rir had already come up with the idea of temporarily storing the previous line

[reply]
      I am trying to parse huge files. I will be looking for a delimiter in each line. When i encounter a delimiter i
      am passing the current line and the filehandle to a subroutine to parse next few lines. But if the next
      lines are incomplete or is not what i am looking for, i have to rollback. so that the parent subroutine will
      continue checking from the line it sent to this subroutine.

      Thanks
      Hari
[reply]
        hari,
        What you want to do is create a flag variable as well as tell. This should give you an idea of how to get started.

        The basic logic is this.

      • Store your position in the file with tell and read a line
      • Check to see if it contains data you are looking for
      • If no, reset the flag and move on as normal
      • If yes, check to see if your special flag is set
      • If yes, move on as normal
      • If no, go into sub-routine
      • When sub-routine returns, determine if it was succesful
      • If yes, move on as normal
      • If no, seek back to the position stored with tell, set your skip flag, use the redo command so that you do not actually read the next line in the file

        I am at work or I would actually write the code. If you need more assistance than the logic and the link to work this out, let me know and I will write it tonight when I get home.

        I hope this helps - cheers - L~R

[reply]

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