You should only have to add one line and modify another.

This strongly implies that you have been given an existing program to modify. It's hardly fair to expect us to do the work without giving us the same head start that you had.

Use the $" variable.

This is perhaps the most puzzling part of the question. I'm not sure how using $" is ever going to make this task easier. The obvious solution is something like:

print "# $_" while <>;
[download]

And even a solution that uses $" like:

my @file = <>;
$" = '#';
print "@file";
[download]

will omit the # from the first line of the output.

I strongly suspect that whoever set this homework doesn't really know that they are doing.

Unexpected things can happen with files, so you may find it helpful to use the -w option as mentioned in the section on running Perl programs.

This is more evidence that the question was written by someone with very little knowledge of Perl. The -w option was surplanted by "use warnings" six years ago. You should only be taught about -w as an item of historical interest.

If you really want to learn Perl then give up this course and buy a good book.

davorg++

Concerning the usage of $", one could augment your solution by doing

my @file = <>;
$" = '#';
print "#@file";
#     ^^^
[download]

but that just adds obscurity to an already obscure solution. So I agree with your assessment of the homework-giver.

On a funny sidenote, something like the following also uses $" :)

$" = '#';
print $", $_ while <>;
[download]

-- Hofmator

You've been pointed to nodes that detail that homework questions are not liked here. I found it a neat challenge to write a program that does what you want but is a bit opaque in what it does:

#!/opt/perl/bin/perl -w
no strict;
no warnings;

# Handle data by prefixing
# the string with "#". The string
# is passed in implicitly via $_
sub handle_data { do{eval}->() };

$/ = "";
while (<DATA>) {
    handle_data;
};

__DATA__
# This program was written by Corion
for   # Anonymous Monk - http://www.perlmonks.org
( # see http://perlmonks.org/?node_id=567615
sub { $/ = "\n" }, 
sub { *DATA = *ARGV; $(}) 
    {$_->()};
  sub { *::handle_data = sub { substr $_, $(,$), "#"; print }
};
[download]

Well, in the real world, I'd do it like so:

Whether your instructor has other goals in mind is something else entirely...

Yup. Or:

perl -pe "print '#'"
[download]

See Is PM a good place to get answers for homework?

• Open the file in the emacs editor
• Hit ctrl-home to get to the top
• Hit ctrl-space to start marking a region
• Hit ctrl-end to get to the end
• Hit ESC-x to enter a command
• type comment-region and hit return

Caveat, only works on modes where emacs thinks the comment-char is "#".

C.

Using Komodo I'd:

• ctrl-a
• ctrl-3

although that may not place the # as the first character of each line - it tends to follow indentation and will be the first non-whitespace character on each line

Update Actually ctrl-3 does place a comment character at the start of each selected line. I was confusing the behaviour with Visual Studio which tends to follow indentation when you use ctrl-k ctrl-c.

---

DWIM is Perl's answer to Gödel

In /g?vim/:

:%s//#/
[download]

---

s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/

ctrl-x h # marks the whole buffer
ctrl-x r t # runs the command string-rectangle. Useful when you want to insert something thats not starts a comment

I've not used the string-rectangle command before - thanks for a great tip in an unexpected place!

Now, here are some interesting links:

• http://www.comp.leeds.ac.uk/Perl/filehandling.html
• http://www.expressnewsindia.com/site/hoshangabad/Perl%20tutorial%20File%20handling.htm
• http://vsbabu.org/tutorials/perl/index-2.html

The top link is likely the original.

The last is possibly the best of them. At least it is nicely formatted. It has an attribution at the top which mentions "Nik Silver, at the School of Computer Studies, University of Leeds, UK".

The first of those is the original authored by Nik Silver. As stated at the top of the first page of the tutorial it was written for Perl 4 and is really only of historical interest. Given that however it is a bit of worry that someone is using it as current teaching material.

/J\

open(INFO, "$file"); @line = <INFO>; close(INFO); $" = '#'; print "#@line";