Hey Monks, I need help understanding how to write to a text file

[Anonymous Monk]

Okay, so I've been lurking around for quite a while now, I just started a new job and need to learn Perl! So far, it seems quite wonderful. My question: I'm reading a friend's code and trying to understand how to open and write to a text file. I looked up how it is commonly done, and this seems different.

 
open(DIN, "<$file_name");
    while (<DIN>)
    {
        next unless /(.*)Avg: (.*) V/;
        print OUT "$i, $2 \n";               
    }
    close(DIN);

sorry if my formatting isn't the greatest, I'm trying I promise! Also, what is "DIN" ?? Even google doesn't seem to be helping me there. thanks a ton <3

[neilwatson]

DIN is a file handle. The syntax is actually depreciated. The more correct way is this:

use strict; # always use this for better code.
use warnings; # always use this for more meaningful error messages.
use English;

my $din;

open( $din, "<", $filename )
   or die "Cannot open file, $filename, error: $ERRNO";
while (<$din>) # process file one line at a time
{
   # do stuff hear on each line.
}
close( $din );

[Anonymous Monk]

This code reads a file. DIN is a filehandle, its name can be chosen arbitrarily. This code is a little "older" style Perl, a more "modern" way to write the same thing would be:

open my $din, '<', $file_name or die "failed to open $file_name: $!";

To write to a file, you'd use ">" instead of "<", and then use print to write to the file, as in print $din "some text\n"; Some quick googling brings up this nice tutorial: http://perlmaven.com/writing-to-files-with-perl , and the documentation for open explains that function in detail.

There are several good books that will help greatly in learning Perl, see for example the "Learning Perl" series here: http://shop.oreilly.com/category/browse-subjects/programming/perl.do . If you don't want to spend any money yet, chromatic's "Modern Perl" is available online for free: http://onyxneon.com/books/modern_perl/

[deejed]

Thank you all a ton! I caved and made an account. Guess I'm a monk now.
Anon: that is the site I found and read while googling, I was trying to understand why my friend had his this way. Found out from him this was NOT working either haha. I shall check out those books!

[vinoth.ree]

Don't you get perldoc open ? in google search ?

If FILEHANDLE, in your case its DIN is an undefined scalar variable, a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE(DIN) is an expression, its value is the real filehandle.

Why should I use three-argument open ?

You should use the three-argument version because it protects against files with crazy names. Consider the following example

my $filename = "<file.txt";
open( INPUTFILE, "< $filename" ) or die "$!";

This will interpolate as: open( INPUTFILE, "< <file.txt" ) or die "$!"; So its actually open a file named file.txt instead of one named <file.txt.