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[3], "\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
       lexical scope.

       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, ...
           each @array
           keys @array

           index $string, $substring  # return value is affected
           pos $string
           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
       between operations.

       Error messages that mention array indices use the 0-based index.

       "keys $arrayref" and "each $arrayref" do not respect the current value
       of $[.

       "$[" in perlvar, Array::Base and String::Base.

perl v5.26.1                      2020-10-19                    arybase(3perl)
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