Re: ReadBackwards usage

Hello jayu_rao,

The method File::ReadBackwards->new() takes a filename (i.e., a string) as its first (and only required) argument. But if you first say:

open my $read_log_file, '<', '/home/jay/my_perl_programs/log.txt' or d
+ie $!;
[download]

you create $read_log_file as a file handle, which is something quite different. (It’s like a special kind of pointer that Perl uses to access a file.) You can see this if you try to print it as a string:

16:13 >perl -Mstrict -wE "open(my $fh, '<', '1184_SoPW.pl') or die; sa
+y qq[>$fh<];"
>GLOB(0x7ec4f8)<

16:14 >
[download]

If you then call File::ReadBackwards->new($fh) it will try to access a file named “GLOB(0x7ec4f8)” — and fail, because there is no file of that name.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Comment on Re: ReadBackwards usage Select or Download Code

Replies are listed 'Best First'.

Re^2: ReadBackwards usage
by jayu_rao (Sexton) on Mar 16, 2015 at 07:17 UTC

The reason is that I would have to work with a temporary file and I would not know the name of the file that is created for giving the actual string.

Jay

[reply]

Re^3: ReadBackwards usage

by Athanasius (Archbishop) on Mar 16, 2015 at 07:48 UTC

Hello jayu_rao,

Here are two possible solutions:

(1) Create the temporary file using the core module File::Temp and get the filename together with the handle:

use File::Temp 'tempfile';
...
my ($fh, $filename) = tempfile();
my  $bw = File::ReadBackwards->new($filename)
    or die "Cannot read '$filename' backwards";

while (my $log_line = $bw->readline)
{
    ...
}
[download]

(2) Use the File::ReadBackwards TIED HANDLE INTERFACE, which accepts a filehandle:

my $filename = 'data.txt';

open(my $bw, '<', $filename)
    or die "Cannot open file '$filename' for reading";

tie *$bw, 'File::ReadBackwards', $filename
    or die "Cannot read '$filename' backwards";

while (my $log_line = <$bw>)
{
    ...
}
[download]

(Note the asterisk in tie *$bw — this is required. Also note the two different ways of reading the next line from the file: readline vs. <$bw>. N.B.: they are not interchangeable!)

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

[reply]
[d/l]
[select]