Perl Programming

Links

• Perl Tutorial.

• scalars are the variable and you prefix them with $ like $variablename
• Double Quote strings do the processing of the variables within it.
• Single Quote strings do the literal substitution of the the variables.

$hello='Hello,World';
print "$hello\n";

• Interestingly, I had named that program as hello.py. :) It obsviously would work. (Magic spell perhaps).
• Arrays are denoted by @ character.
• push(@arrayname, “value”) is a good way to add “value” to the array @arrayname.
• push function returns the length of the new list.

@newarray = ("one","two","three");
print push(@newarray);

Output: 3

• just the @arrayname returns the length.
• opposite of push is the pop, which returns the last element.
• having the array within ” “ processes the values and returns them space separated. The delimiter can be changed by changing $” variable.

@foodlist = ("dosa","idly","parota","kesari","poori");
print push(@foodlist)."\n";
print pop(@foodlist)."\n";
print @foodlist."\n";
$val1 = @foodlist;
print $val1."\n";
$"='-';
$val2 = "@foodlist";
print $val2."\n";

OUTPUT:
5
poori
4
4
dosa-idly-parota-kesari

• The index of the last element can be got by $#@arrayname.
• To read from a file, perl uses angled brackets.

$file = '/etc/passwd';
open(INFO, "$file");
@lines = <INFO>;
close(INFO);
print @lines;

• Do while loop, getting from the STDIN.

print "Password? ";
$a = <STDIN>;
chop $a;

while ($a ne "Senthil")
{
        print "Sorry, try again:";
        $a = <STDIN>;
        chop $a;
}
print "Good Guess: $a \n";

• chop function removes the last character of the string.
• Programming Exercise. Appending # in front using $” and appending line numbers, 1,2,3.. and 01,02,..039 while printing lines from a file.

$file = '/etc/passwd';
open(INFO, "$file");
@lines = <INFO>;
close(INFO);
$"="#";
print "#@lines";

$lineno = "0001";
open(INFO,"$file");
while ($line = <INFO>)
{
        print "$lineno $line";
        $lineno++;
}
close(INFO);

• One of the most useful features of Perl is its powerful string matching facilities.
• Regular expression is contained within /’s and matching occurs with =~ operator.
• =~ is match, !~ is does not match.
• $_ is the current string, which /regex/ would act upon.

$_ = "Sharma is in Australia right now, probably playing rugby";

if (/Australia/)  # This is the match expression.
{
        print "Down under";
}

• In regex, b stands for word boundary. get the meaning of it properly.
• Special read only variables, $`, $& and $’ represents what was matched before, during and after the search.
• Associative Arrays are like dictionaries.
• Unlike list array, the associative arrays are indexed by {} probably because they are fancier.

%associative = ("one",1,
                "two",2,
                "three",3);

print $associative{"one"}."\n";

@listarray = %associative;

print "@listarray\n";
print $listarray[0]."\n";
print $listarray[1]."\n";

%newassociative = @listarray;

print $newassociative{"three"}."\n";

• there is keys function, values function and each function (key, value).
• functions are called subroutines.
• the subroutine is called with the & character in front of its name.
• Parameters are passed in the special @_ list variable and individual elements can be accessed by $_[index] value.

sub mysub
{
        print "Hello,World\n";
}

&mysub;

sub mysub2
{
        print @_;
        print $_[1];
}

&mysub2("Hello",1,"World");

• Result of the subroutine is always the last thing that was evaluated. Its fun! No return statement. In case of print as the last it is 1.
• variables can be made local with the local function.

sub inside
{
        local($a,$b) = ($_[0], $_[1]);
        $a =~ s/\ //g;
        $b =~ s/\ //g;
        (($a =~ /$b/) || ($b =~ /$a/));
}

print &inside("henry", "Man and his hen rye");

• The rule is simple: in Perl, parentheses for built-in functions are never required nor forbidden. Their use can help or hinder clarity, so use your own judgment.
• qw - quote each of the words.
• whitespace is generally insignificant in any perl programs.
• my defines the variables inside it to be private within the enclosing block.