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Programming Perl

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Functions
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3.2.87 local

local EXPR

This operator declares one or more global variables to have locally scoped values within the innermost enclosing block, subroutine, eval, or file. If more than one variable is listed, the list must be placed in parentheses, because the operator binds more tightly than comma. All the listed variables must be legal lvalues, that is, something you could assign to. This operator works by saving the current values of those variables on a hidden stack and restoring them upon exiting the block, subroutine, or eval, or file. After the local is executed, but before the scope is exited, any called subroutines will see the local, inner value, not the previous, outer value, because the variable is still a global variable, despite having a localized value. The technical term for this is "dynamic scoping".

The EXPR may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given, all scalars are initialized to the undefined value and all arrays and hashes to empty.) Commonly, this is used to name the formal arguments to a subroutine. As with ordinary assignment, if you use parentheses around the variables on the left (or if the variable is an array or hash), the expression on the right is evaluated in list context. Otherwise the expression on the right is evaluated in scalar context.

Here is a routine that executes some random piece of code that depends on $i running through a range of numbers. Note that the scope of $i propagates into the eval code.

&RANGEVAL(20, 30, '$foo[$i] = $i');

sub RANGEVAL {
    local($min, $max, $thunk) = @_;
    local $result = "";
    local $i;

    # Presumably $thunk makes reference to $i

    for ($i = $min; $i < $max; $i++) {
        $result .= eval $thunk;
    }

    $result;
}

This code demonstrates how to make a temporary modification to a global array:

if ($sw eq '-v') {
    # init local array with global array
    local @ARGV = @ARGV;
    unshift @ARGV, 'echo';
    system @ARGV;
}
# @ARGV restored

You can also temporarily modify hashes:

# temporarily add a couple of entries to the %digits hash
if ($base12) {
    # (NOTE: not claiming this is efficient!)
    local(%digits) = (%digits, T => 10, E => 11);
    parse_num();
}

But you probably want to be using my instead, because local isn't really what most people think of as local. See the section on my later.


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