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

Hi monks,
I am trying to read 3 files at a time,below is my code.
#!/usr/bin/perl
open(F1,"tax.pl");
while($line=<F1>)
{
    chomp $line;
    @arr=split(/"\n"/,$line);
     print "@arr\n";
}
open(F2,"accno.pl");
{
while($line1=<F2>)
    {
    chomp $line;
    @arr1=split(/"\n"/,$line1);
    print "@arr1\n";
    }
}
 open(F3,"ptax.pl");
 while($line=<F3>)
 {
     chomp $line;
     @arr3=split(/"\n"/,$line);
     print "@arr3\n";
 }

[download]

The above code is reading all the contents of 3 files, i required to read only the last lines from these file, how can we read only the last lines from these files.
keep pouring your ideas.

Replies are listed 'Best First'.
Re: how can i read only the last line of a files
by davorg (Chancellor) on Aug 03, 2006 at 09:53 UTC
[reply]
      Hi, If you are trying to read last line of continously updated files (like weblog file), I think your best choice would be File::Tail inconjuction with Proc::Daemon. Thanks, Uday.
[reply]
Re: how can i read only the last line of a files
by liverpole (Monsignor) on Aug 03, 2006 at 11:37 UTC
    Here's an alternative way to do it.

    The function last_n_lines takes 3 arguments:  the filename, the number of lines N to read from the end, and an optional maximum line length.  It then returns a reference to an array containing the last N lines from the file.  It will work especially well on very huge files, as it doesn't read all of the lines into memory first: 

    #!/usr/bin/perl -w
                                                                      
+          
# Strict
use strict;
use warnings;
                                                                      
+          
# User-defined
my $n    = 3;                   # How many lines to read from the end
my $file = "/usr/dict/words";   # File to read
                                                                      
+          
                                                                      
+          
# Main program
my $plines = last_n_lines($file, $n, 256);
print "Last $n line(s) from '$file':\n";
for (my $i = 0; $i < @$plines; $i++) {
    printf " %2d.  '%s'\n", $i+1, $plines->[$i];
}
                                                                      
+          
                                                                      
+          
#
#  In:  $1 ... name of file
#       $2 ... number of lines to read
#       $3 ... (optional) maximum line length
#
# Out:  $1 ... pointer to list of the last N lines from the file.
#
sub last_n_lines {
    my ($fname, $nlines, $maxlen) = @_;
                                                                      
+          
    # Open the file
    open(my $fh, "<", $file) or die "Error -- failed to read '$fname' 
+($!)\n";
                                                                      
+          
    # Calculate offset near end of file
    $maxlen ||= 1024;
    my $offset = (-s $file) - ($maxlen + 1) * $nlines;
    ($offset > 0) and seek($fh, $offset, 0);
                                                                      
+          
    # Read lines
    my @lines;
    while (!eof($fh)) {
        chomp(my $line = <$fh>);
        push @lines, $line;
    }
                                                                      
+          
    # Return last N
    close $fh;
    splice(@lines, 0, -$nlines);
    return \@lines;
}

    [download]

    Update:  Removed a line which I just realized I had put in during testing of the program.

    s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/
[reply]
[d/l]
Re: how can i read only the last line of a files
by rodion (Chaplain) on Aug 03, 2006 at 10:24 UTC
    I think your best bet is to give GrandFather enough information to do something clever, elegant, and efficient. Pending that, here's my contributuion to getting the last line from each of the three files in a fairly basic way. 
    my $last_line;
for my $fname ( 'tax.pl', 'accno.pl', 'ptax.pl' ) {
  open INFILE, $fname;
  while (<INFILE>) {
    chomp;
    $last_line = $_;
  }
  close INFILE;
  print "$fname:$last_line\n";
  $last_line = '';   # in case 2nd or 3rd file is empty
}

    [download]
    This will work with huge files, where you don't want to store the whole file in memory, although, by the extensions, I'm guessing your files are fairly short. Also, for huge files, you almost certainly want File::ReadBackwards as davorg has already suggested.
[reply]
[d/l]
Re: how can i read only the last line of a files
by GrandFather (Saint) on Aug 03, 2006 at 10:10 UTC

    Is this a one time thing, or are the files updated from time to time and you need to retreive material added to the end?

    Is there a maximum line length?

    Are the files vary large?

    Is each file the same length?

    DWIM is Perl's answer to Gödel
[reply]
Re: how can i read only the last line of a files
by perladdict (Chaplain) on Aug 03, 2006 at 10:03 UTC
    hi monk,
    i saw your code,u are reading the content of files in an array.below subroutine may helps to read the last content of arrays.
    @last=lastlines(\@a,\@b,\@c);
print "@last\n";
sub lastlines
{
    my @retlist=();
     foreach my $aref(@_)
    {
         push @retlist,pop @$aref;
    }
    return @retlist;
}

    [download]
    .
[reply]
[d/l]
      This is equivalent code (though note davorg's reply for a much more robust solution) ... no need to iterate through everything: 
      sub lastlines
{
  return map { $_[-1] } @_;
}

# or, w/o using a sub:
@last = ($a[-1], $b[-1], $c[-1]);

      [download]
[reply]
[d/l]
Re: how can i read only the last line of a files
by sh1tn (Priest) on Aug 03, 2006 at 21:32 UTC
    In addition - see eof 
    eof # this is the last line
 and print while <$fh>

    [download]


[reply]
[d/l]

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