How to match previous line

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

I am attempting to pull a number (in my example 1530631) from a line before the line with the beep. * Line that needs to be matched, and I need 1530631. (8/20/09 7:59:28 PM EDT) 1530631: ^^5 * Than I need to match this line and pull this number 1531639. (8/20/09 7:59:28 PM EDT) 1531639: ##Twenty Four;##;pp * I will take the two numbers and subtract them from each other. My current code can only pull the line with the beep, but i cannot figure how to match the line before the beep.

 
# open a file
my $filename = $ARGV[0];
open(debugfile, $filename) || die "Sorry, couldn't open file!";

while($line = <debugfile>)
{
    
    if($line =~ /##BEEP/) # place the regular expression to match the 
+first line here
    {
        print $line;
        @firstLine = split(/\)/, $line);
        $firstLineTime = $firstLine[1];
        @match = split (/##/, $firstLineTime);
        $Tic = $match[0];
        $waitingForTargetStatement = 1;
        
#        print "$firstLineTime\n";
#        print "$Tic\n";
        next;
    }


    if($line =~ /Pick/ && $waitingForTargetStatement == 1) # place the
+ regular expression to match the second line here
    {
        $waitingForTargetStatement = 0;
        
        @secondLine = split(/\)/, $line);
        $secondLineTime = $secondLine[1];
        @match = split (/##/, $secondLineTime);
        $Tic = $match[0];
        
        $time = $secondLineTime - $firstLineTime;
        $time = $time/1000;
        
        print $line;
#        print "$Tic\n";
        print "The time between the beep and the next pick location is
+ $time seconds\n"; # modify the print statement
        print "\n";
        next;
    }
#    if($line =~ /##|\^\^|SYSMSG|DIALOG/) # place a regular expression
+ to include other desired content here.
#    {
#        print $line;
#    }
}
[download]

EXAMPLE OF LOG:

(8/20/09 7:59:28 PM EDT) 1524243: ##BEEP (375,2)##
(8/20/09 7:59:28 PM EDT) 1524516: ##Two;Five;   Pick 
(8/20/09 7:59:28 PM EDT) 1530337: ^^7                     
(8/20/09 7:59:28 PM EDT) 1530631: ^^5                    
(8/20/09 7:59:28 PM EDT) 1531465: ##BEEP (375,2)##
(8/20/09 7:59:28 PM EDT) 1531639: ##Twenty Four;##;pp
(8/20/09 7:59:44 PM EDT) 1534041: ^^ready                
(8/20/09 7:59:44 PM EDT) 1534177: ##Three;One;   Pick 
(8/20/09 7:59:44 PM EDT) 1540464: ^^8                    
(8/20/09 7:59:44 PM EDT) 1540760: ^^8                    
(8/20/09 7:59:44 PM EDT) 1541590: ##BEEP (375,2)##
(8/20/09 7:59:44 PM EDT) 1541884: ##Two;Three;   Pick
[download]

Comment on How to match previous line Select or Download Code

Replies are listed 'Best First'.
Re: How to match previous line by moritz (Cardinal) on Aug 26, 2009 at 16:17 UTC
You need something along the lines of `use strict; use warnings; my $previous; while (my $line = <$handle>) { if (defined($previous) && $line =~ /BEEP/ ) { print $previous; } $previous = $line; }` [download] Of course you need additional code to extract the number from `$previous`, but that doesn't seem to be a problem for you. Update: as ambrus++ pointed out in the CB this is a good place to use a continue block for assigning to `$previous`. Perl 6 projects - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
Re^2: How to match previous line by JavaFan (Canon) on Aug 26, 2009 at 16:33 UTC
Considering that $previous is defined most of the time, it's better to check `$line =~ /BEEP/` first. Or just eliminate the check for definedness all together: `my $previous = ""; while (<$handle>) { print $previous if /BEEP/; $previous = $_; }` [download] If "BEEP" is on the first line, it just prints nothing. Which is the same effect as not printing (assuming no assignment to `$\` has been done).	[reply] [d/l] [select]
Re: How to match previous line by wagnerc (Sexton) on Aug 26, 2009 at 17:05 UTC
Hi. Three ways to do it. One, slurp the file and parse it using subscripts. So when `$line[$i]` matches, u can pull `$line[$i - 1]` to get the other value. Two, save the current line value into a variable which will be available on the next iteration of the loop. `while ($line = <FILE>) { if ($line is good) { dosomething($line, $lastline); } $lastline = $line; }` [download] Three, use Tie::File.	[reply] [d/l] [select]