CODEWITHSUNDEEP
perl
Leon WANG
Leon WANG

Reputation: 103

How to use variable instead of a file handle

I have a big data file dump.all.lammpstrj which I need to split/categorize into a series of files, such as Z_1_filename, Z_2_filename, Z_3_filename etc. based on the coordinates in each record.

The coordinates are saved in a disordered way, so my program reads each line and determines which file this record should be sent to.

I use a variable, $filehandle = "Z_$i_DUMP"

and I hope to open all of the possible files like this

for ( my $i = 1; $i <= 100; $i++ ) {
  $filehandle = "Z_$i_DUMP";
  open $filehandle,'>', "Z_$i_dump.all.lammpstrj.dat";
  ...
}

But when running my program, I get a message

Can't use string ("Z_90_DUMP") as a symbol ref while "strict refs" in use at ...

I don't want to scan all the data for each output file, because dump.all.lammpstrj is so big that a scan would take a long time.

Is there any way to use a defined variable as a file handle?

Upvotes: 3

Views: 2928

Answers (3)

Leon WANG
Leon WANG

Reputation: 103

I sincerely appreciate Kjetil S. and sticky bit. I tested it and their suggestion work well. And I noticed that there is another way to write data to different files WITHOUT CHANGING filehandler. Actually I changed file names using same file handler. 

....
for my $i=0;$i<=$max_number;$i++;
{
   $file="$i\_foo.dat";
   open DAT,'>>',"$file";
   ......
}

Upvotes: 0

Kjetil S.
Kjetil S.

Reputation: 3787

To give you an idea on how this might be done. Put file handles in a hash (or perhaps array if indexed by numbers).

use strict;
use warnings;
my %fh;                                                        #file handles
open $fh{$_}, '>', "Z_${_}_dump.all.lammpstrj.dat" for 1..100; #open 100 files
for(1..10000){                                    #write 10000 lines in 100 files
    my $random=int(1+rand(100));                  #pick random file handle
    print {$fh{$random}} "something $_\n";
}
close $fh{$_} for 1..100;

Upvotes: 5

sticky bit
sticky bit

Reputation: 37487

Don't assign anything to $filehandle or set it to undef before you call open(). You get this error because you have assigned a string to $filehandle (which is of no use anyway).

Also see "open" in perldoc:

If FILEHANDLE is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.)

To have more file handles at a time and to conveniently map them to the file names consider using a hash with the file name (or whatever identifier suits you) as key to store them in. You can check if the key exists (see "exists") and the value is defined (see "defined") to avoid reopening the file unnecessarily.

Upvotes: 2

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