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

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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);
[download]

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

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Re: Hey Monks, I need help understanding how to write to a text file by neilwatson (Priest) on Jun 11, 2014 at 18:01 UTC
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 );` [download] Neil Watson watson-wilson.ca	[reply] [d/l]
Re: Hey Monks, I need help understanding how to write to a text file by Anonymous Monk on Jun 11, 2014 at 18:09 UTC
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: $!";` [download] 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/`	[reply] [d/l] [select]
Re: Hey Monks, I need help understanding how to write to a text file by deejed (Initiate) on Jun 11, 2014 at 18:18 UTC
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!	[reply]
Re: Hey Monks, I need help understanding how to write to a text file by vinoth.ree (Monsignor) on Jun 11, 2014 at 18:20 UTC
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, "< $filename" ) or die "$!";` [download] 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. *All is well*	[reply] [d/l] [select]