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.IX Title "Perl::Critic::Policy::Variables::RequireNegativeIndices 3pm"
.TH Perl::Critic::Policy::Variables::RequireNegativeIndices 3pm "2020-05-17" "perl v5.30.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
Perl::Critic::Policy::Variables::RequireNegativeIndices \- Negative array index should be used.
.SH "AFFILIATION"
.IX Header "AFFILIATION"
This Policy is part of the core Perl::Critic
distribution.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Perl treats a negative array subscript as an offset from the end. Given
this, the preferred way to get the last element is \f(CW$x[\-1]\fR, not
\&\f(CW$x[$#x]\fR or \f(CW$x[@x\-1]\fR, and the preferred way to get the next-to-last
is \f(CW$x[\-2]\fR, not \f(CW\*(C`$x[$#x\-1\*(C'\fR or \f(CW$x[@x\-2]\fR.
.PP
The biggest argument against the non-preferred forms is that \fBtheir
semantics change\fR when the computed index becomes negative. If \f(CW@x\fR
contains at least two elements, \f(CW$x[$#x\-1]\fR and \f(CW$x[@x\-2]\fR are
equivalent to \f(CW$x[\-2]\fR. But if it contains a single element,
\&\f(CW$x[$#x\-1]\fR and \f(CW$x[@x\-2]\fR are both equivalent to \f(CW$x[\-1]\fR. Simply
put, the preferred form is more likely to do what you actually want.
.PP
As Conway points out, the preferred forms also perform better, are more
readable, and are easier to maintain.
.PP
This policy notices all of the simple forms of the above problem, but
does not recognize any of these more complex examples:
.PP
.Vb 2
\&    $some\->[$data_structure]\->[$#{$some\->[$data_structure]} \-1];
\&    my $ref = \e@arr; $ref\->[$#arr];
.Ve
.SH "CONFIGURATION"
.IX Header "CONFIGURATION"
This Policy is not configurable except for the standard options.
.SH "AUTHOR"
.IX Header "AUTHOR"
Chris Dolan <cdolan@cpan.org>
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2006\-2011 Chris Dolan.
.PP
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.