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Variables: Difference between revisions
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→{{header|Perl}}
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=={{header|Perl}}==
Variables can be declared with <code>our</code>, <code>my</code>, or <code>local</code>, or they can be used without being declared at all; see [[scope modifiers]] for the differences. In any case, variables which haven't been assigned to have the undefined value by default. The undefined value acts just like <code>0</code> (if used as a number) or the empty string (if used as a string), except it can be distinguished from either of these with the <code>defined</code> function. Also, if warnings are enabled, perl will print a message like "Use of uninitialized value $foo in addition (+)" whenever you use the undefined value as a number or string.▼
In perl, variables are global by default and can be manipulated from anywhere in the program. Variables can be used without first being declared, unless the strict pragmatic directive is in effect:
<lang perl>sub dofruit {
$fruit='apple';
}
dofruit;
print "The fruit is $fruit";</lang>
▲Variables can be declared prior to use and may be prefixed with [[scope modifiers]] <code>our</code>, <code>my</code>, or <code>local</code>
Initialization and assignment are the same thing in Perl: just use the <code>=</code> operator. Note that the rvalue's context (scalar or list) is determined based on the lvalue.
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There are two other sigils, but they behave quite unlike the others. A token of the form <code>&foo</code> refers to a subroutine named <code>foo</code>. In older versions of Perl, ampersands were necessary for calling user-defined subroutines, but since they no longer are, they have only a handful of obscure uses, like making references to named subroutines. Note that you can't assign to an ampersand-marked name. But you can assign to a typeglob, a kind of object represented with the notation <code>*var</code>. A typeglob <code>*foo</code> represents the symbol-table entry for all of the otherwise independent variables <code>$foo</code>, <code>@foo</code>, <code>%foo</code>, and <code>&foo</code>. Assigning a string <code>"bar"</code> to <code>*foo</code> makes these variables aliases for <code>$bar</code>, <code>@bar</code>, <code>%bar</code>, and <code>&bar</code> respectively. Alternatively, you can assign a reference to a typeglob, which creates an alias only for the variable of the appropriate type. In particular, you can say <code>*twiddle = sub {...}</code> to change the definition of the subroutine <code>&twiddle</code> without affecting <code>$twiddle</code> and friends.
If the strict pragmatic directive is in effect, then variables need explicit scope declaration, so should be prefixed with a my or our keyword depending on the required level of scope:
<lang perl>use strict;
our $fruit; # declare a variable as global
our $veg = "carrot"; # declare a global variable and define its value</lang>
=={{header|Perl 6}}==
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