zapdos has asked for the wisdom of the Perl Monks concerning the following question:

So I'm following the book Learning Perl, there's this code with comments in it: 
$rocks[0] = 'bedrock';
$rocks[1] = 'slate';
$rocks[2]= 'lava';
$rocks[3] = 'crushed rock';
$rocks[99] = 'schist';

$#rocks = 2; # forget all rocks after 'lava'
$#rocks = 99; # add 97 undef elements (the forgotten rocks are gone fo
+rever)

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If I do print $rocks[$#rocks]; it prints nothing. Why? When I comment out $#rocks = 99; it prints 'lava' and when I comment out $#rocks = 2; it prints 'schist'. But when I maintain the two it prints nothing as I already said. And what the comments "add 97 undef elements (the forgotten rocks are gone forever)" and "forget all rocks after 'lava'" mean?

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Re: What's going on in array element assignment?
by AnomalousMonk (Archbishop) on Aug 11, 2020 at 02:17 UTC
[reply]
[d/l]
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Re: What's going on in array element assignment?
by perlfan (Parson) on Aug 11, 2020 at 02:36 UTC
    This paints a better picture: 
    use strict;
use warnings;
use Data::Dumper ();

my @rocks = ();
$rocks[0] = 'bedrock';
$rocks[1] = 'slate';
$rocks[2]= 'lava';
$rocks[3] = 'crushed rock';

print Data::Dumper::Dumper(\@rocks);

$rocks[99] = 'schist';

print Data::Dumper::Dumper(\@rocks);

$#rocks = 2; # forget all rocks after 'lava'
print Data::Dumper::Dumper(\@rocks);

$#rocks = 99; # add 97 undef e
print Data::Dumper::Dumper(\@rocks);

    [download]
    Also, if you want to do surgery on your array, look at splice.
[reply]
[d/l]
      $rocks[0] = 'bedrock';
$rocks[1] = 'slate';
$rocks[2]= 'lava';
$rocks[3] = 'crushed rock';
$rocks[99] = 'schist';

$#rocks = 2; 
$#rocks = 99; 
print $rocks[$#rocks];

      [download]
      So print $rocks[$#rocks]; is printing nothing because the last element is undef?
[reply]
[d/l]
[select]
        Yes, $#rocks = 2; made your array unaware of subsequent elements (though the memory may still be intact, but I don't think the underlying datastructure can be reverted easily); however $#rocks = 99; probably overwrote the memory forever with undefs. I don't know what memory managment magic is invoked here, but I think on a high level that's an accurate way to think about it.
[reply]
[d/l]
[select]

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