How can I print a matching line, one line immediately above it and one line immediately below?

Question

From a related question asked by Bi, I\'ve learnt how to print a matching line together with the line immediately below it. The code looks really simple:

#!p         


        

Answer 1

Here's a modernized version of Pax's excellent answer:

use strict;
use warnings;

open( my $fh, '<', 'qq.in') 
    or die "Error opening file - $!\n";

my $this_line = "";
my $do_next = 0;

while(<$fh>) {
    my $last_line = $this_line;
    $this_line = $_;

    if ($this_line =~ /XXX/) {
        print $last_line unless $do_next;
        print $this_line;
        $do_next = 1;
    } else {
        print $this_line if $do_next;
        $last_line = "";
        $do_next = 0;
    }
}
close ($fh);

See Why is three-argument open calls with lexical filehandles a Perl best practice? for an discussion of the reasons for the most important changes.

Important changes:

• 3 argument open.
• lexical filehandle
• added strict and warnings pragmas.
• variables declared with lexical scope.

Minor changes (issues of style and personal taste):

• removed unneeded parens from post-fix if
• converted an if-not contstruct into unless.

If you find this answer useful, be sure to up-vote Pax's original.