arybase(3perl) Perl Programmers Reference Guide arybase(3perl)
arybase - Set indexing base via $[
$[ = 1;
@a = qw(Sun Mon Tue Wed Thu Fri Sat);
print $a, "\n"; # prints Tue
This module implements Perl's $[ variable. You should not use it
Assigning to $[ has the compile-time effect of making the assigned
value, converted to an integer, the index of the first element in an
array and the first character in a substring, within the enclosing
It can be written with or without "local":
$[ = 1;
local $[ = 1;
It only works if the assignment can be detected at compile time and the
value assigned is constant.
It affects the following operations:
splice @array, $index, ...
index $string, $substring # return value is affected
substr $string, $offset, ...
As with the default base of 0, negative bases count from the end of the
array or string, starting with -1. If $[ is a positive integer,
indices from "$[-1" to 0 also count from the end. If $[ is negative
(why would you do that, though?), indices from $[ to 0 count from the
beginning of the string, but indices below $[ count from the end of the
string as though the base were 0.
Prior to Perl 5.16, indices from 0 to "$[-1" inclusive, for positive
values of $[, behaved differently for different operations; negative
indices equal to or greater than a negative $[ likewise behaved
Before Perl 5, $[ was a global variable that affected all array indices
and string offsets.
Starting with Perl 5, it became a file-scoped compile-time directive,
which could be made lexically-scoped with "local". "File-scoped" means
that the $[ assignment could leak out of the block in which occurred:
$[ = 1;
# ... array base is 1 here ...
# ... still 1, but not in other files ...
In Perl 5.10, it became strictly lexical. The file-scoped behaviour
was removed (perhaps inadvertently, but what's done is done).
In Perl 5.16, the implementation was moved into this module, and out of
the Perl core. The erratic behaviour that occurred with indices
between -1 and $[ was made consistent between operations, and, for
negative bases, indices from $[ to -1 inclusive were made consistent
Error messages that mention array indices use the 0-based index.
"keys $arrayref" and "each $arrayref" do not respect the current value
"$[" in perlvar, Array::Base and String::Base.
perl v5.22.1 2020-10-19 arybase(3perl)
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