File handle question

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

Monks,

I have a very silly and simple question, but i dont understand why it works like this.

First while loop working properly whereas the second is not, why?

undef $/;
open (FCMP, "anchor.txt") or die ("anchor.txt is not opened\n");
{
    local $/="\n";
    
    while(<FCMP>)
    {
        print "$_\n";
        chomp;
        @item = split(/\t/);
        $pii{$item[0]} = [$item[1], $item[2], $item[3]];

    }
}
{
local $/="\n";

while (<FCMP>)
{
my $an = $_;
print "$an\n";
}
}
[download]

The second while statement is not printing anything. So i thought that only one time a file handle can be used directly <FCMP> without assigning to a variable. Is it so? If wrong where am i going wrong.

Regards,
Anniyan
(CREATED in HELL by DEVIL to s|EVILS|GOODS|g in WORLD)

Comment on File handle question Download Code

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Re: File handle question by Corion (Patriarch) on Aug 12, 2005 at 06:48 UTC
Once you step through a file, you need to rewind it (if possible) to read through it again. Your first loop steps through the file to the end, so the second loop doesn't have any lines to step over. You can either reopen `FCMP`, or rewind it to position 0: `# open/reopen my $filename = "anchor.txt"; open FCMP, "<", $filename or die "Couldn't open '$filename': $!"; # or, alternatively seek FCMP,0,0; # rewind` [download]	[reply] [d/l]
Re: File handle question by Zaxo (Archbishop) on Aug 12, 2005 at 06:47 UTC
You need to seek back to the beginning of the file. The first loop exhausted it, so the second begins at eof(). After Compline, Zaxo	[reply]
Re: File handle question by hatter (Pilgrim) on Aug 12, 2005 at 12:15 UTC
If you start off with `open (FCMP, "anchor.txt") or die ("anchor.txt is not opened\n"); my @file = <FCMP>;` [download] you can then replace `while(<FCMP>)` with `foreach (@file)` and the replace the second loop in its entirety, with `print join("\n",@file)` If you want bonus cleverness, you could even replace the first loop with a single `map()` command, but that's probably leaping a step or two up on the learning curve. Either way, as others have said, using <> steps through each line of input, you can either seek or re-open the file to go back to the beginning, or simply read the file into an array first, which avoids having to read the file from disk an extra time (a more expensive operation) `the hatter`	[reply] [d/l] [select]
Re: File handle question by murugu (Curate) on Aug 12, 2005 at 09:23 UTC
Hi, In the first loop itself eof(end of the file) is reached. So in second loop nothing is getting printed. Regards, Murugesan Kandasamy.	[reply]