Re^2: Merging files

Wow this is so cool! Thanks Boris/C/Mirod!

I intially thought I will just learn how to open multiple filehandles but got lured into implementing the paste command like program (not there yet :)).. Here is my code but for some reason my $line variable does not get set to undef when 'all' filehandles runs out of data...Should i be checking for something else to terminate the loop?

I guess the code will be much faster if i can generate perl code that puts all the filehandles into one line without looping them through it...

#! /usr/local/bin/perl -w

# Open many file handels

# C's code
foreach my $file (@ARGV) {
   my $fh;
   open $fh, "<", $file or die "Can't open $file ($!)";
   push @filehandles,  $fh;
}

while (1) {
    $line = undef; # Not sure whether this is even required.
    foreach (@filehandles) {
        $line .= <$_>;
        chomp($line);
    }
    print ($line,"\n");
    last if undef($line);
}
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Re^3: Merging files by borisz (Canon) on Apr 11, 2005 at 08:36 UTC
Since the lhs of `.=` is always a defined value, even if you append `undef`. `perl -MData::Dumper -e '$x.= undef; print Dumper($x)' __OUTPUT__ $VAR1 = '';` [download] Boris	[reply] [d/l]
Re^3: Merging files by mirod (Canon) on Apr 11, 2005 at 08:45 UTC
I think you should keep track of which filehandle is still open, and read it only in that case. And I am always wary of `while(1)` loops. But its probably just me ;--) `#!/usr/bin/perl -w use strict; use Fatal qw(open close); my @files= @ARGV; my %fh; # file => file handle foreach my $file (@files) { open( my $fh, '<', $file); $fh{$file}= $fh; } while( keys %fh) { foreach my $file (@files) { if( exists $fh{$file}) { my $fh= $fh{$file}; if( defined( my $line= <$fh>)) { chomp $line; print $line;} # regular line else { delete $fh{$file}; } # eof reached for this file } } print "\n"; # end of line for all files }` [download]	[reply] [d/l]
Re^4: Merging files by sk (Curate) on Apr 11, 2005 at 09:23 UTC
Thanks again! Now i understand the undef issue. Also thanks for the one-liner, showed me how to use Dumper!!! Mirod, I agree, it is definitely better to keep track of filehandles and close them when they run of out data! Learnt a few nice things today :)	[reply]
Re^4: Merging files by cazz (Pilgrim) on Apr 11, 2005 at 14:26 UTC
I agree, while(1) loops can be bad. I'm fond of doing something similar to what you are doing instead of while(1). I have spent much more time than I care to remember debugging code that was due to poor while(1) implementations. I have a few minor nits to pick with your example. You don't close any of your file handles. Why keep em around once you are done? I would inject a close right before your delete Intead of repeatedly checking every file in the list you are processing, why not just check the list of files currently open? The foreach my $file (@files) loop is easy to swap out with a foreach my $fh (keys %fh) The change would look something like this: `while (keys %fh) { foreach my $fh (keys %fh) { if (defined(my $line = <$fh>)) { chomp $line; print $line; } else { close($fh); delete($fh{$fh}); } } }` [download]	[reply] [d/l]