Perl::Critic::Policy::Variables::RequireNegativeIndices3pm

Affiliation

This Policy is part of the core Perl::Critic distribution.

Overview

Perl treats a negative array subscript as an offset from the end. Given this, the preferred way to get the last element is $x[-1], not $x[$#x] or $x[@x-1], and the preferred way to get the next-to-last is $x[-2], not "$x[$#x-1" or $x[@x-2].

The biggest argument against the non-preferred forms is that their semantics change when the computed index becomes negative. If @x contains at least two elements, $x[$#x-1] and $x[@x-2] are equivalent to $x[-2]. But if it contains a single element, $x[$#x-1] and $x[@x-2] are both equivalent to $x[-1]. Simply put, the preferred form is more likely to do what you actually want.

As Conway points out, the preferred forms also perform better, are more readable, and are easier to maintain.

This policy notices all of the simple forms of the above problem, but does not recognize any of these more complex examples:

    $some->[$data_structure]->[$#{$some->[$data_structure]} -1];
    my $ref = \@arr; $ref->[$#arr];

Configuration

This Policy is not configurable except for the standard options.