Variable Variables

Get introduced to variable variables in this lesson and learn how you can use them in Perl.

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What are variable Variables?

Using Perl, we can access data through dynamic variable names. The name of a variable can be stored in another variable, allowing it to be accessed dynamically. Such variables are known as variable variables.

To turn a variable into a variable variable, you put an extra $ sign in front of your variable. This method is illustrated in the following figure:

Note: Variable name, say $vehicle, can be put between {}.We will see its usage in the next section.

Implementation

The following code snippet shows how this is done in Perl:

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$vehicle = "car"; # vehicle has value "car"
$$vehicle = "honda"; # car has value "honda"
print "\$vehicle:\t"; 
print $vehicle; #prints car
print "\n";
print "\${\$vehicle}:\t";   # using \ as $ is special character
print ${$vehicle}; #prints honda
print "\n";
print "\$\$vehicle:\t"; 
print $$vehicle; #prints honda
print "\n";
print "\$car:\t\t"; 
print $car; #prints honda
print "\n";

The following example shows two different ways of accessing a variable’s value: directly and using variable variables. It displays the text “Blue” on the screen using both ways:

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$skyColor = 'Blue';
$varPrefix = 'sky';
print "\$skyColor:\t\t";
print $skyColor; # Outputs "Blue"
print"\n";
print "\${\$varPrefix . 'Color'}:\t"; 
=POD
varPrefix will give value sky and the dot operator concatenates
it with Color hence making it skyColor. 
Putting $ before skycolor makes it a variable variable.
=cut
print ${$varPrefix . 'Color'}; # Also outputs "Blue"

Using {} is only mandatory when the name of the variable is itself an expression and . operator is used to concatenating it with other variables part to get the result, like this:

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${$variableNamePart1 . $variableNamePart2} = $value;

Don’t worry about the (.) operator, we’ll discuss that in details in the string chapter.

It is nevertheless recommended to always use {} because it’s more readable.

Quick quiz

Q

What will be the output of the following code?

  $fruit = "Mango";
  $$fruit = "yellow";
	print "The color of ", $fruit , " is "  , $Mango ;
A)

Compile time error

B)

The color of fruit is yellow

C)

The color of Mango is yellow