arybase - Set indexing base via $[

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You should not use it directly.</p> <p>Assigning to <code>$[</code> has the <i>compile-time</i> effect of making the assigned value, converted to an integer, the index of the first element in an array and the first character in a substring, within the enclosing lexical scope.</p> <p>It can be written with or without <code>local</code>:</p> <pre><code>$[ = 1; local $[ = 1;</code></pre> <p>It only works if the assignment can be detected at compile time and the value assigned is constant.</p> <p>It affects the following operations:</p> <pre><code>$array[$element] @array[@slice] $#array (list())[$slice] splice @array, $index, ... each @array keys @array index $string, $substring # return value is affected pos $string substr $string, $offset, ...</code></pre> <p>As with the default base of 0, negative bases count from the end of the array or string, starting with -1. If <code>$[</code> is a positive integer, indices from <code>$[-1</code> to 0 also count from the end. If <code>$[</code> is negative (why would you do that, though?), indices from <code>$[</code> to 0 count from the beginning of the string, but indices below <code>$[</code> count from the end of the string as though the base were 0.</p> <p>Prior to Perl 5.16, indices from 0 to <code>$[-1</code> inclusive, for positive values of <code>$[</code>, behaved differently for different operations; negative indices equal to or greater than a negative <code>$[</code> likewise behaved inconsistently.</p> <h1 id="HISTORY"><a class="permalink" href="#HISTORY">#</a>HISTORY</h1> <p>Before Perl 5, <code>$[</code> was a global variable that affected all array indices and string offsets.</p> <p>Starting with Perl 5, it became a file-scoped compile-time directive, which could be made lexically-scoped with <code>local</code>. "File-scoped" means that the <code>$[</code> assignment could leak out of the block in which occurred:</p> <pre><code>{ $[ = 1; # ... array base is 1 here ... } # ... still 1, but not in other files ...</code></pre> <p>In Perl 5.10, it became strictly lexical. The file-scoped behaviour was removed (perhaps inadvertently, but what's done is done).</p> <p>In Perl 5.16, the implementation was moved into this module, and out of the Perl core. The erratic behaviour that occurred with indices between -1 and <code>$[</code> was made consistent between operations, and, for negative bases, indices from <code>$[</code> to -1 inclusive were made consistent between operations.</p> <h1 id="BUGS"><a class="permalink" href="#BUGS">#</a>BUGS</h1> <p>Error messages that mention array indices use the 0-based index.</p> <p><code>keys $arrayref</code> and <code>each $arrayref</code> do not respect the current value of <code>$[</code>.</p> <h1 id="SEE-ALSO"><a class="permalink" href="#SEE-ALSO">#</a><a id="SEE"></a>SEE ALSO</h1> <p><a href="/5.26.3/perlvar#%24%5B">"$[" in perlvar</a>, <a href="/5.26.3/Array::Base">Array::Base</a> and <a href="/5.26.3/String::Base">String::Base</a>.</p> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). 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arybase - Set indexing base via $[ - Perldoc Browser