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arybase(3)	       Perl Programmers	Reference Guide		    arybase(3)

       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 v5.28.3			  2020-05-14			    arybase(3)


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