Multiple simultaneous filehandles

[ezekiel]

I have a tab-delimited text file of an unspecified number (say x) columns. I want to split this into x-1 separate tab-delimited text files where the first file contains just the first and second columns, the second file contains the first and third columns, etc.

As I don't know the number of columns in advance, the filehandles for the output files have to be variables. So I have the following code:

use strict;

my $number_of_files = 0;

while (<STDIN>) {
    chomp;
    
    my @line = split;
    
    # On the first list, set up the process
    
    if ($. == 1) {
        for (my $i = 1; $i < @line; $i++) {
        
            my $fh = "OUTPUT" . $i;
            my $filename = $line[$i] . ".txt";
                
            open ($fh, ">$filename") or die "Cannot open the output fi
+le: $!";
                        
        }
        
        $number_of_files = @line;
        
    }
    
    else {
        for (my $i = 1; $i < @line; $i++) {
        
            my $fh = "OUTPUT" . $i;
            print $fh ("$line[0]\t$line[$i]\n");
            
        }
        
    }
    
}

for (my $i = 1; $i <= $number_of_files; $i++) {
    my $fh = "OUTPUT" . $i;
    close $fh or die "Cannot close the output file: $!";
            
}

The problem is I get a "Can't use string ("OUTPUT1") as a symbol ref while "strict refs" in use" error. Where have I gone wrong?

[gaal]

You're trying to use talk about the filehandle symbolically, that is, construct its name:

my $fh = "OUTPUT" . $i;

Don't do that. Since version 5.6, Perl allows you to conveniently put filhandles in lexically scoped variables. In your case you're indexing by a number, so an array of handles is probably the best choice.

# before the if ($. == 1)
my @fhs;

...

# open one
open $fhs[$i], ">", $filename or die "Cannot open the output file: $!"
+;

...

# use it
print {$fhs[$i]} "$line[0]\t$line[$i]\n";

Note it's important to control the scope of @fhs, so it's visible to your code that uses it. When it goes out of scope, the files are closed.

Update: fixed syntax error, thanks eyespoplikeamosquito++!

[eyepopslikeamosquito]

# use it
print $fhs[$i] "$line[0]\t$line[$i]\n";

That is a syntax error. Better style is:

print {$fhs[$i]} "$line[0]\t$line[$i]\n";

See Perl Best Practices, Chapter 10, "Printing to Filehandles".

Update: Or, if you prefer OO-style IO:

use IO::Handle;
# ...
$fhs[$i]->print("$line[0]\t$line[$i]\n");

[grep]

There is little (unless these files are huge) need to make this so complicated. Just create the structure of the file in a AoAoA and process. No need to worry about multiple file handles, just use one over and over again. Whenever I tackle something like this, I almost always create the data structure and then process.

If you're having a tough time coming up with a solution, it's almost always a good idea to visulize/write down the data structure. Then the program makes itself. Oh and Data::Dumper is your friend.

use strict;
use warnings;

my @file = (
'1    2    3    4',
'5    6    7    8',
'9    10    11    12',
'13    14    15    16');

my @data;
foreach my $line (@file) {
    my @fields = split(/\t/,$line);
    for (my $x = 1 ; $x <= $#fields ; $x++) {
        push @{$data[$x-1]},  [$fields[0],$fields[$x]];
    }
    
}

for ( my $x = 0 ; $x <= $#data; $x++) {
    open(FH,">OUTPUT_$x") or die "$!\n";
    foreach my $fields ( @{$data[$x]} ) {
        print FH "$fields->[0] $fields->[1]\n";
    }
    close FH;
}

[blazar]

The problem is I get a "Can't use string ("OUTPUT1") as a symbol ref while "strict refs" in use" error. Where have I gone wrong?

Others already told you what you're doing wrong, and I also concur with grep that you're making things overly complicated. I want to contribute, and here's how I'd do it:

#!/usr/bin/perl

use strict;
use warnings;

chomp(my $line=<>);
my @line=split /\t/, $line;

{
    my @fh=map {
        my $fname=sprintf 'output%02d.txt', $_;
        open my $fh, '>', $fname or
          die "Can't open `$fname': $!\n";
        $fh } 1..$#line;
    
    sub doline {
        my $one=shift;
        print { $fh[$_] } "$one\t$_[$_]\n" for 0..$#_;
    }
}
            
doline @line;
chomp, doline split /\t/ while <>;

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