1arybase(3pm)           Perl Programmers Reference Guide           arybase(3pm)
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NAME

6       arybase - Set indexing base via $[
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SYNOPSIS

9           $[ = 1;
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11           @a = qw(Sun Mon Tue Wed Thu Fri Sat);
12           print $a[3], "\n";  # prints Tue
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DESCRIPTION

15       This module implements Perl's $[ variable.  You should not use it
16       directly.
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18       Assigning to $[ has the compile-time effect of making the assigned
19       value, converted to an integer, the index of the first element in an
20       array and the first character in a substring, within the enclosing
21       lexical scope.
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23       It can be written with or without "local":
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25           $[ = 1;
26           local $[ = 1;
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28       It only works if the assignment can be detected at compile time and the
29       value assigned is constant.
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31       It affects the following operations:
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33           $array[$element]
34           @array[@slice]
35           $#array
36           (list())[$slice]
37           splice @array, $index, ...
38           each @array
39           keys @array
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41           index $string, $substring  # return value is affected
42           pos $string
43           substr $string, $offset, ...
44
45       As with the default base of 0, negative bases count from the end of the
46       array or string, starting with -1.  If $[ is a positive integer,
47       indices from "$[-1" to 0 also count from the end.  If $[ is negative
48       (why would you do that, though?), indices from $[ to 0 count from the
49       beginning of the string, but indices below $[ count from the end of the
50       string as though the base were 0.
51
52       Prior to Perl 5.16, indices from 0 to "$[-1" inclusive, for positive
53       values of $[, behaved differently for different operations; negative
54       indices equal to or greater than a negative $[ likewise behaved
55       inconsistently.
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HISTORY

58       Before Perl 5, $[ was a global variable that affected all array indices
59       and string offsets.
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61       Starting with Perl 5, it became a file-scoped compile-time directive,
62       which could be made lexically-scoped with "local".  "File-scoped" means
63       that the $[ assignment could leak out of the block in which occurred:
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65           {
66               $[ = 1;
67               # ... array base is 1 here ...
68           }
69           # ... still 1, but not in other files ...
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71       In Perl 5.10, it became strictly lexical.  The file-scoped behaviour
72       was removed (perhaps inadvertently, but what's done is done).
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74       In Perl 5.16, the implementation was moved into this module, and out of
75       the Perl core.  The erratic behaviour that occurred with indices
76       between -1 and $[ was made consistent between operations, and, for
77       negative bases, indices from $[ to -1 inclusive were made consistent
78       between operations.
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BUGS

81       Error messages that mention array indices use the 0-based index.
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83       "keys $arrayref" and "each $arrayref" do not respect the current value
84       of $[.
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SEE ALSO

87       "$[" in perlvar, Array::Base and String::Base.
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91perl v5.16.3                      2013-03-04                      arybase(3pm)
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