How to get previous line of the file while processing current line in while loop

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

Hi Monks,

I want to know the previous line of the file while processing it in current line in while loop. For example see the below code

    my $file = 'input.txt';
    open STDLOG, "$file" or die("$file file cannot be opened: $!\n");

    while (defined(my $row = <STDLOG>)) {
        chomp $row;
        print "Current line is: $row \n";
        # here i want to print the previous line 
    }
[download]

Note: Input file is not stored in array, since it is big file.

Thanks in advance
Shanmugam A.

Comment on How to get previous line of the file while processing current line in while loop Download Code

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Re: How to get previous line of the file while processing current line in while loop by Nikhil Jain (Monk) on Mar 29, 2011 at 07:37 UTC
`use strict; use warnings; open(my $fh, '<', "$file") or die $!;#use three argument open my $previous_line = q{}; #initially previous line would be empty while(my $current_line = <$fh>){ chomp; print"$current_line\n"; print"$previous_line\n"; #assign current line into previous line before it go to next line $previous_line = $current_line; } close($fh);` [download]	[reply] [d/l]
Re: How to get previous line of the file while processing current line in while loop by JavaFan (Canon) on Mar 29, 2011 at 09:17 UTC
Your question is "how do I implement a single element buffer"? It's easy: `my $previous = <STDLOG>; while (my $current = <STDLOG>) { print "Current line is: $current"; print "Previous line is $previous"; $previous = $current; }` [download]	[reply] [d/l]
Re: How to get previous line of the file while processing current line in while loop by Anonymous Monk on Mar 29, 2011 at 15:01 UTC
Possible heresy If its just for a trivial *nix sysadmin task, you could do worse than `man grep` and check out some options including `-A`	[reply] [d/l] [select]