http://www.perlmonks.org?node_id=1074405


in reply to Reading file and matching lines

In Perl, you can read a file line by line without the need to load the whole file first. Use the diamond operator in a while loop (untested code): 
open my $IN, '<', 'quoteout.dat' or die "$!";

my $searching_for_G;

while (<$IN>) {
    $searching_for_G = 1      if 0 == index $_, 'E';
    die "Error: h at line $." if $searching_for_G and 0 == index $_, '
+h';
    if ($searching_for_G and 0 == index $_, 'G') {
        print "Found G at line $.\n";
        undef $searching_for_G;
    }
}

[download]

Note that unlike in C, a string is not an array of characters in Perl (that's why I used index). Also, you did not specify what to do if G is found - should the program end or search for another E? I assumed the latter.

$. contains the input line number. See perlvar for details.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

Replies are listed 'Best First'.
Re^2: Reading file and matching lines
by robby_dobby (Hermit) on Feb 11, 2014 at 12:56 UTC
    Wouldn't it be better if you had just used $searching_for_G and /^h/? (I haven't tested this). It may well just be me, but 0 == index ... sticks out oddly to my eyes.
[reply]
[d/l]
[select]
Re^2: Reading file and matching lines
by Jalcock501 (Sexton) on Feb 11, 2014 at 13:26 UTC
    Apologies, Yes there are several instances of this in a single file, so I need to do this through out the file and only report any errors if there are any. If none the script should exit normally.
[reply]
      "...only report any errors if there are any"

      If so, why not something simple like this:

      while (<IN>) {
    print qq($1\n) if /^(E)/;
    print qq($1\n) if /^(G)/;
    die $1 if /^(h)/;
}

      [download]

      Or do i still misunderstand the specs?

      Best regards, Karl

      «The Crux of the Biscuit is the Apostrophe»

[reply]
[d/l]
      use strict;
use warnings;
my $infile  = shift;
my $found_E = 0;
my $sets    = 0;
open my $ifh, '<', $infile;
while(<$ifh>) {
  if (/^E/) {
    $found_E = 1;
    next;
  }
  if ($found_E) {
    if (/^G/) {
       $sets += 1;
       $found_E = 0;
       next;
    }
    if (/^h/) {
      print "Error! Found h before G\n";
      exit;
    }
  }
}
close($ifh);
printf "Found %d sets from E to G uninterrupted by h\n",$sets;

      [download]
[reply]
[d/l]

In Section Seekers of Perl Wisdom