Perl Cheatsheet

This covers Perl 5. Also see my regular expressions cheatsheet.

Modules
Arrays
Hashes
Conditionals and Loops
Files
Subroutines
Math
Magic variables

Modules

Check if a module is installed by trying to open its documentation:

perldoc GD::Graph

Install modules using the CPAN shell:

perl -MCPAN -e shell

Find a module in CPAN:

cpan> i /GD/

Install a module using CPAN:

cpan> install GD::Graph

Arrays

Initialize (clear) an array:

my @array = ();

Assign values to an array:

@array = ("foo", "bar", "bat");

Get value of an array element:

print $array[1];

Pushing and popping elements to or from the end of an array:

push (@array, $newElement);
$x = pop (@array);

The functions shift() and unshift() do the same thing as push() and pop(), but to the beginning of an array.

Find length of array:

$arraySize = scalar (@array);

or:

$arraySize = $#array + 1;

Arrays can be sorted in a variety of ways:

@reversedArray = reverse (@array);
@sortedAscendingArray = sort {$a <=> $b} @array;
@sortedDescendingArray = sort {$b <=> $a} @array;

Hashes

Create a hash:

my %hash = ();
%hash = (
	key0	=>	"value foo",
	key1	=>	"value bar",
	key2	=>	"value bat"
);

Use hash:

print $hash{key1};

Check if hash element exists:

if (exists ($hash{'key1'})) { print "Hash key1 exists."; }

Add to a hash:

$hash{$key3} = "value bam";

Remove hash element:

delete ($hash{'key3'});

Iterate over a hash:

while (($key, $value) = each (%hash)) { print "$key and $value.\n"; }

Another couple of ways to loop over hashes:

foreach $key (keys %hash) { print $hash{$key}; }
foreach $value (values %hash) { print $value; }

Conditionals and Loops

If/else conditionals:

if ($n == 1) {
	print "It's 1.\n";
} elsif ($n == 2) {
	print "It's 2.\n";
} else {
	print "It's not 1 or 2.\n";
}

for loop:

for ($i = 0; $i < 10; $i ++) {
	print "Count is $i.\n";
}

The foreach loop, looping over elements in an array:

 foreach $element (@array) {
	print $element;
}

Files

Open a file, and read it:

open (INPUT, "<$inputFile")
	or die ("Can't open read input file $inputFile");
while (<INPUT>) {
	print $_;
}
close (INPUT);

Open a file, and write to it:

open (OUTPUT, ">$outputFile")
  or die ("Can't open output file $outputFile");
print OUTPUT $output;
close (OUTPUT);

Subroutines

sub mysub {
	print "First subroutine argument is $_[0]\n";
}

Math

Round to two decimal places:

$rounded = sprintf ("%.2f", 100.23456);

Magic variables

This list is not exhaustive, but these are the ones I commonly use:

$_	Current default value
$!	Current error
@ARGV	Command line arguments
@_	Subroutine arguments $_[0], $_[1], etc.
$1	$2, $3, etc.; Pattern matches
$∓	Last pattern match
$.	Current input line number