rjc33 has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I have a tab-delimited file in which some lines are completely blank, some have an "a" in the first column and others have an "s" in the first column. I want to print all lines to an output file, except in cases where one or more "s" lines directly follow another "s" line - if this happens then I only want to print the first "s" line. I thought the best way to do this would be to loop through two lines at a time and say unless both the current line and the previous line start with a "s" the line should be printed. I've played with many different ways of writing this, but something like this is probably simplest: 

use warnings;
use strict;

open (IN, "in.txt") or die;
open (OUT, ">out.txt") or die;

my $previous_line = <IN>;

while (my $current_line = <IN>) {
    chomp for ($previous_line, $current_line);
    unless ( ($current_line =~ /^s/) and ($previous_line =~ /^s/) ) {
        print OUT "$current_line\n";
    }
}

close IN;
close OUT;

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This prints all of the lines in the file, would be very grateful if someone could explain why? If there's a completely different way to tackle this then it would be great to hear as well, I'm new to coding so this just seemed the most logical way to do it. Cheers!
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Re: Issue with unless statement with two conditions acting on two lines
by toolic (Bishop) on Jan 08, 2016 at 15:49 UTC
    One problem is that $previous_line is always the 1st line of the file. You need to update it in the loop.
[reply]
      Wow, that's embarrassing! Thanks a lot, works just fine now - been a long day staring at a screen, can't believe I left that off and didn't realise. Cheers!
[reply]
        print all lines to an output file
        But not the first one ?
        poj
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Re: Issue with unless statement with two conditions acting on two lines
by hippo (Archbishop) on Jan 08, 2016 at 16:01 UTC 
    use strict;
use warnings;

my $saw_an_s = 0;
while (my $current_line = <DATA>) {
    next if ($saw_an_s and $current_line =~ /^s/);
    $saw_an_s = $current_line =~ /^s/ ? 1 : 0;
    print $current_line;
}
__DATA__
s 1
s 2
s 3
a

a foo
s 4
s 5
s 6
a bar

    [download]

    Just keep a flag rather than store the whole of the previous line each time - much simpler (to my mind). HTH.

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      Thanks for the suggestion, I'll try this in the future!
[reply]

        Note that hippo's approach above also prints the first line without further effort.

        Give a man a fish:  <%-{-{-{-<

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[d/l]
Re: Issue with unless statement with two conditions acting on two lines
by Corion (Patriarch) on Jan 08, 2016 at 15:49 UTC

    Have you inspected/printed out the values of $previous_line and $current_line while your program goes through the loop?

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[d/l]
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