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Perl Cookbook by Nathan Torkington, Tom Christiansen

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Exchanging Values Without Using Temporary Variables

Problem

You want to exchange the values of two scalar variables, but don’t want to use a temporary variable.

Solution

Use list assignment to reorder the variables.

($VAR1, $VAR2) = ($VAR2, $VAR1);

Discussion

Most programming languages force you to use an intermediate step when swapping two variables’ values:

$temp    = $a;
$a       = $b;
$b       = $temp;

Not so in Perl. It tracks both sides of the assignment, guaranteeing that you don’t accidentally clobber any of your values. This lets you eliminate the temporary variable:

$a       = "alpha";
$b       = "omega";
($a, $b) = ($b, $a);        # the first shall be last -- and versa vice

You can even exchange more than two variables at once:

($alpha, $beta, $production) = qw(January March August);
# move beta       to alpha,
# move production to beta,
# move alpha      to production
($alpha, $beta, $production) = ($beta, $production, $alpha);

When this code finishes, $alpha, $beta, and $production have the values "March“, "August“, and "January“.

See Also

The section on “List value constructors” in perldata(1) and Chapter 2 of Programming Perl

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