Perl File Handling

Question

The below is the Perl script that I wrote today. This reads the content from one file and writes on the other file. It works but, not completely.

#---------------------------------------------------------------------------
#!/usr/bin/perl

open IFILE, "text3.txt" or die "File not found";
open OFILE, ">text4.txt" or die "File not found";

my $lineno = 0;

while(<IFILE>)
{
 @var=<IFILE>;
 $lineno++;
 print OFILE "@var";
}

close(<IFILE>);
close(<OFILE>);
#---------------------------------------------------------------------------

The issue is, it reads and writes contens, but not all. text3.txt has four lines. The above script reads only from second line and writes on text4.txt. So, finally I get only three lines (line.no 2 to line.no 4) of text3.txt.

What is wrong with the above program. I don't have any idea about how to check the execution flow on Perl scripts. Kindly help me.

I'm completely new to Programming. I believe, learning all these would help me in changing my career path.

Thanks in Advance,

Vijay

Answer 1

<IFILE> reads one line from IFILE (only one because it's in scalar context). So while(<IFILE>) reads the first line, then the <IFILE> in list context within the while block reads the rest. What you want to do is:

# To read each line one by one:
while(!eof(IFILE)) { # check if end of file is reached instead of reading a line
  my $line = <IFILE>; # scalar context, reads only one line
  print OFILE $line;
}

# Or to read the whole file at once:
my @content = <IFILE>; # list context, read whole file
print OFILE @content;

Answer 2

The problem is that this line...

while(<IFILE>)

...reads one line from text3.txt, and then this line...

@var=<IFILE>;

...reads ALL of the remaining lines from text3.txt.

You can do it either way, by looping with while or all at once with @var=<IFILE>, but trying to do both won't work.

Answer 3

This is how I would have written the code in your question.

#!/usr/bin/perl
use warnings;
use strict;
use autodie;

# don't need to use "or die ..." when using the autodie module
open my $input,  '<', 'text3.txt';
open my $output, '>', 'text4.txt';

while(<$input>){
  my $lineno = $.;
  print {$output} $_;
}
# both files get closed automatically when they go out of scope
# so no need to close them explicitly

I would recommend always putting use strict and use warnings at the beginning of all Perl files. At least until you know exactly why it is recommended.

I used autodie so that I didn't have to check the return value of open manually. ^{_{( autodie was added to Core in version 5.10.1 )}}

I used the three argument form of open because it is more robust.

It is important to note that while (<$input>){ ... } gets transformed into while (defined($_ = <$input>)){ ... } by the compiler. Which means that the current line is in the $_ variable.

I also used the special $. variable to get the current line number, rather than trying to keep track of the number myself.

Answer 4

When you're closing the files, use just

close(IFILE);
close(OFILE);

When you surround a file handle with angle brackets like <IFILE>, Perl interprets that to mean "read a line of text from the file inside the angle brackets". Instead of reading from the file, you want to close the actual file itself here.

Answer 5

There is a couple of questions you might want to think about, if you are strictly copying a file you could use File::Copy module.

If you are going to process the input before writing it out, you might also consider whether you want to keep both files open at the same time or instead read the whole content of the first file (into memory) first, and then write it to the outfile.

This depends on what you are doing underneath. Also if you have a huge binary file, each line in the while-loop might end up huge, so if memory is indeed an issue you might want to use more low-level stream-based reading, more info on I/O: http://oreilly.com/catalog/cookbook/chapter/ch08.html

My suggestion would be to use the cleaner PBP suggested way:

#!/usr/bin/perl

use strict;
use warnings;
use English qw(-no_match_vars);

my $in_file  = 'text3.txt';
my $out_file = 'text4.txt';

open my $in_fh,  '<', $in_file  or die "Unable to open '$in_file': $OS_ERROR";
open my $out_fh, '>', $out_file or die "Unable to open '$out_file': $OS_ERROR";

while (<$in_fh>) {
    # $_ is automatically populated with the current line
    print { $out_fh } $_ or die "Unable to write to '$out_file': $OS_ERROR";
}

close $in_fh  or die "Unable to close '$in_file': $OS_ERROR";
close $out_fh or die "Unable to close '$out_file': $OS_ERROR";

OR just print out the whole in-file directly:

#!/usr/bin/perl

use strict;
use warnings;
use English qw(-no_match_vars);

my $in_file  = 'text3.txt';
my $out_file = 'text4.txt';

open my $in_fh,  '<', $in_file  or die "Unable to open '$in_file': $OS_ERROR";
open my $out_fh, '>', $out_file or die "Unable to open '$out_file': $OS_ERROR";

local $INPUT_RECORD_SEPARATOR; # Slurp mode, read in all content at once, see: perldoc perlvar

print { $out_fh } <$in_fh> or die "Unable to write to '$out_file': $OS_ERROR";;

close $in_fh  or die "Unable to close '$in_file': $OS_ERROR";
close $out_fh or die "Unable to close '$out_file': $OS_ERROR";

In addition if you just want to apply a regular expression or similar to a file quickly, you can look into the -i switch of the perl command: perldoc perlrun

perl -p -i.bak -e 's/foo/bar/g' text3.txt; # replace all foo with bar in text3.txt and save original in text3.txt.bak