Perl Coding Hints

[ Much of this information came from perldoc.com.]

Case Conversion

Use the uc and lc functions to convert text to uppercase or lowercase. For example, $tmp = uc($tmp); will make the text in $tmp all uppercase.

Writing Files

It's IMPOSSIBLE to open a file and insert text at the beginning or middle of the file, without overwriting text that's already in the file. This is a limitation of computer file systems, not Perl -- no language can do this. You need to use some kind of technique like writing your output to a temp file, then overwriting the original file with the temp file when you're done. The PerlFAQ 5 has this covered, in a rather assuming-you-already-know-the-answer way.

Random How-Tos

To round a number to 3 decimal places:

use POSIX;
$score = sprintf "%.3f", ($hits/$total)*100;



Hash sorting: you can't actually sort a hash
itself, but you can sort one INTO an array:

# Hash sorted by keys:
@keys = sort keys %SomeHash;
# Hash sorted in reverse by keys:
@keys = sort { $b cmp $a } keys %SomeHash;
# Hash sorted by values:
@values = sort values %SomeHash;
# Hash sorted in reverse by values:
@values = sort { $b cmp $a } values %SomeHash;



To sort a hash in ascending order by its values and print
the associated keys in the same sorted order with the values:
( from http://www.devdaily.com/perl/edu/qanda/plqa00016/ )

foreach $key (sort { $myhash{$a} <=> $myhash{$b} } (keys(%myhash)))
{ print "$myhash{$key}, $key"; }
# Note that $a and $b are global variables, you don't need to set them.
# And, just switch the $a and $b to sort in descending order.

This will sort by keys, and populate an array instead of printing:

foreach $key (sort (keys(%myhash)))
{ push @sortedHash, "$myhash{$key}, $key"; }



To get the IP address when you know the hostname:
use Socket;
my $ipaddress = inet_ntoa(scalar gethostbyname("www.google.com"));

To get the hostname when you know the IP address:
use Socket;
my $hostname = gethostbyaddr(inet_aton("216.239.33.101"), AF_INET);

Or use one of these scripts, which are longer but don't require any external modules like Socket:
   iphostqueries (command prompt version)
   iphostqueries (web-based version)



To cause your fatal error messages (die) go to the browser, in addition to the logfile:
use CGI::Carp qw(fatalsToBrowser);



Strip leading spaces from a string:  $string =~ s/^\s+//;
Strip trailing spaces from a string: $string =~ s/\s+$//;



To find the current working directory:
On Linux: my $wd = `cwd`;
On Windows: use Cwd; my $wd = cwd;



Chop off the first 4 characters from the string in $a:
$a = substr($a, 4);
Save only the first 4 characters from the string in $a:
$a = substr($a, 0, 4);
Chop off the last 4 characters from the string in $a:
$a = substr($a, 0, -4);
Save only the last 4 characters from the string in $a:
$a = substr($a, -4);



To expand the variables within another variable:
$a =~ s/(\$\w+)/$1/eeg;
die if $@;

This is useful when you use this format for defining a variable:

$a = <<'STOPHERE';

this stuff will be inside my variable.
$me will be too!
"You can use quotes without escaping them when you declare
a variable this way."

STOPHERE



To detect resolution on Windows:
use Win32::GUI;
$Main = Win32::GUI::Window->new(-name => 'Main',-text => 'Perl',);
$DesktopWin = $Main->GetDesktopWindow();
(undef,undef,$DesktopWidth,$DesktopHeight) = Win32::GUI::GetWindowRect+($DesktopWin);
print "Screen Resolution is $DesktopWidth x $DesktopHeight";



To create a new process / "fork" a new process on Windows, and then return control
to your script without waiting for the new process to complete, use the
Windows start command along with the Perl system() function:

system("start /B someprogram.exe") and die "$0: $!\n";
# do other stuff here

someprogram.exe will run, and your script will immediately pick up doing other stuff,
without having to wait for someprogram.exe to finish whatever it's doing. Note that
the /B switch to the start command causes someprogram.exe to run without opening a
new command-prompt window.

To install perl modules, read this.