Re^3: accessing variable vaule outside for loop

In Perl arrays, the index '-1' accesses the last element in the array. '-2' accesses the second-to-last. ...and so on, down to '- scalar @array', which will be the same element as '$array[0]'.

This makes it easy to simultaneously count upward and downward; just do a numeric negation of sign, and adjust for off-by-one.

@numbers = ( 100, 200, 300, 400, 500 );

$reversed[$_-1] = $numbers[-$_] for 1 .. @numbers;

print "$_\n" for @reversed;
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Here's just about the only useful use of $[ (which should be avoided, even here).

@numbers = ( 100, 200, 300, 400, 500 );

{
  no warnings 'deprecated';
  local $[ = 1;             # Don't do this... example only.

  $reversed[$_] = $numbers[-$_] for 1 .. @numbers;
}

print "$_\n" for @reversed;
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Dave

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Re^4: accessing variable vaule outside for loop by AnomalousMonk (Archbishop) on Jul 07, 2013 at 00:59 UTC
The array index negate-adjust trick (easily encapsulated in a subroutine) is particularly useful when dealing with 'large' arrays, the reversed elements of which can then be accessed in-place: no reversed copy, possibly quite expensive in terms of space, need be made.	[reply]