find a match and pull nth line before the match

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

Hi folks, I'm looking to read in a file, search for a particular string and then pring the nth line before the line containing the match. The code below which prints lines before and after the match are getting to where I want to be, but I'm looking for something cleaner/easier or other better approaches to this. Thanks in advance!

use strict;
use warnings;
use Tie::File;
sub max ($$) { $_[$_[0] < $_[1]] }
sub min ($$) { $_[$_[0] > $_[1]] }
my $file_in = 'data.txt';
my @lines;
tie @lines, 'Tie::File', $file_in or die "Unable to tie $file_in $!";
my $n_recs = @lines; # how many records are in the file?
my @lnbrs = ();
my $i = 0;
while ($i < $n_recs - 1) {
    my $twolines = $lines[$i] . $lines[$i + 1];
    if ($twolines =~ m/file\s+for\s+(?:bankrup|chapter)/im) {
        push @lnbrs, $i
    }
    $i++;
}

foreach my $ln (@lnbrs) {
    my $start = max($ln-3, 0);
    my $end = min ($ln+3, $n_recs);
    <strong class="highlight">print</strong> '-' x 75, "\n";
    foreach ($start .. $end) {
        <strong class="highlight">print</strong> "$lines[$_]\n";
    }
}
untie @lines;
[download]

Comment on find a match and pull nth line before the match Download Code

Replies are listed 'Best First'.
Re: find a match and pull nth line before the match by JavaFan (Canon) on Mar 14, 2012 at 02:09 UTC
Untested: `my $N = ...; # Number of lines before matching line. my $PATTERN = "..."; # What you want to match. my @buffer; while (my $line = <>) { if (@buffer < $N) { push @buffer, $line; next; } if ($line =~ /$PATTERN/) { print $buffer[0]; } shift @buffer; push @buffer, $line; }` [download]	[reply] [d/l]
Re^2: find a match and pull nth line before the match by msensay (Novice) on Mar 14, 2012 at 19:31 UTC
Thanks JavaFan. That's exactly what I needed.	[reply]
Re: find a match and pull nth line before the match by BrowserUk (Patriarch) on Mar 14, 2012 at 02:20 UTC
`perl -nE"push @b,$_;say $b[0] if /pattern/;shift@b if @b >10;" file.in` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l]