Perl Variables

Describes the three types of Perl variables and provides examples of Perl variable use.

Variable Indications and types

Three types of perl variables are:

  1. Scalar designated by $.
  2. Array designated by @.
  3. Hash or associative array indicated by %.

The "$" sign indicates scalar variables. They do not need to be declared before they are used. For instance the line:

$myprog = "/usr/bin/progname";

Sets the variable, $myprog to the string value of "/usr/bin/progname" which is the path and name of a program. The line:

$j = 10;

sets the variable $j to a value of 10.

Variable Manipulation

Strings

The "." symbol or period causes one string to be added or concatenated to another. Therefore given the following example:

$X = "Hi ";
$Y = "there!";
$X = $X . $Y;

The last statement will take the contents of $X and $Y and place them in $X which is now "Hi there!". A shorthand way to add them is:

$X .= $Y;

This statement performs the same function as the third line above. The value of $X can be seen with the following command:

print $X;




Arrays

Arrays are created as follows:

@tags = ( 'FORM', 'TABLE', 'OL', 'UL');

The statement:

print $tags[0, "\n"];

prints the string "FORM". The string table is referred to with the $tags[1] reference and so on. The statement:

Print $#tags, "\n";

Will print the largest index value of the array which is 3 in this case, therefore looping for all values in the array may be done as follows:

for ($i = 0; $i < = $#tags; $i++)
{
   print $tags[$i], "\n";
}

This will print all the values in the array on separate lines. Another way to do this is:

foreach $i (@tags)
{
   print $i, "\n";
}

The "foreach" command will place each element of the array @tage in $i until all elements have been used.

Hash

Hashes are similar to arrays (hashes are also called associative arrays), but contain the data in pairs called a KEY and associated VALUE. Hashes are designated with the '%' sign rather than the '@' as in an array. The {} brackets are used to reference elements in the hash rather than the [] brackets as in normal arrays. Please note that the () brackets are used to create the hash or array.

%group = ('forest', 'tree', 'crowd', 'person');
print "A $group{'forest'} is in the forest.\n";  #A tree is in a forest.

Adding elements to a hash can be done with either of the following methods:

%group = (%group, 'fleet', 'ship');
$group{'herd'}='cow';

A list argument can be used to create the hash as follows:

%group = (
	forest -> 'tree',
	crowd -> 'person',
	fleet -> 'ship',
	herd -> 'cow'
);

Two hash variables may be combined into one as follows:

%group1 = ('forest', 'tree', 'crowd', 'person');
%group2 = ('herd', 'cow', 'fleet', 'ship');
%group = (%group1, %group2);

A hash can be printed as follows:

print @{[%group]}\n;