Hello good Monks,

I am having difficulty parsing a huge data file. Since it is a huge file, I can only read line-by-line (with the exception of the small buffer I'm using).

The rule I am trying to implement has these preconditions (please refer to the @sourcedata array below):
  1. A literal "1" occurs at the beginning of the line, (and is the only char on that line)
  2. ... and is immediately followed by *any* number of newlines
  3. ... and is terminated by the literal "_____ 2"

IFF these conditions are met, then the newlines between the "1" and "_____ 2" lines are removed. Any other condition just prints everything (including the buffer.)

For some reason, I cannot execute the loop where I am buffering the intermediate newlines.

Also please note that for ease of this discussion, I have described this question in terms of arrays, rather than file I/O -- this is not germaine to the solution I am seeking.

Here's some sample data

#!/usr/bin/perl

use strict;
use warnings;

# NOTE: The @sourcedata array is the representation of 
# the data as if it were read from a file by:
#
#   open(FH, $sourcefilename) || die ...
#   my @sourcedata = <FH>;
#   close FH;
#
# Since the source file is huge, I need to process the
# file line by line

# NOTE: I updated this array to reflect an array of lines
my @sourcedata = (
"\n",
"1\n",
"\n",
"\n",
"b\n",
"\n",
"1\n",
"\n",
"\n",
"\n",
"\n",
"_____ 2\n",
"\n",
"\n",
"\n"
);

# The desired result of processing a small data sample:
my @desiredoutput = qq(

1

b

1
_____ 2    # NOTE: preceding newlines have been collapsed



);

[download]

Here's my source code...


my @buffer = ();
my $length = scalar @sourcedata;

for (my $I = 0; $I < $length; $I++) {
  my $line = $sourcedata[$I];
  if ($line =~ /^1$/) {
    push(@buffer, $line);
    $I++;
    $line = $sourcedata[$I];

    # Here's the loop I can't seem to execute:
    while ($line =~ /^\n$/ && $I != $length) {
      print "Buffering...\n";
      push(@buffer, $line);
      $I++;
      last if $I == $length;
      $line = $sourcedata[$I];
    }
    if ($line =~ /_____ 2/) {
      # print only the first and last items in the buffer,
      # effectively removing the empty lines
      print shift(@buffer), pop(@buffer);
      print $line;
    }
    else {
      print join(@buffer);
    }
  }
  else {
    print $line;
  }
  @buffer = ();
}

[download]
Where do you want *them* to go today?

In reply to Implementing a parsing rule in a huge data file by thezip

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.