What's going on in array element assignment?

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)
[download]

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?

Comment on What's going on in array element assignment? Select or Download Code

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Re: What's going on in array element assignment? by AnomalousMonk (Archbishop) on Aug 11, 2020 at 02:17 UTC
Please show your code. Please see Short, Self-Contained, Correct Example. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l]
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Re: What's going on in array element assignment? by perlfan (Vicar) 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); $#rocks = 99; # add 97 undef e print Data::Dumper::Dumper(\@rocks);` [download] Also, if you want to do surgery on your array, look at splice.	[reply] [d/l]
Re^2: What's going on in array element assignment? by zapdos (Sexton) on Aug 11, 2020 at 02:53 UTC
`$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]
Re^3: What's going on in array element assignment? by perlfan (Vicar) on Aug 11, 2020 at 03:20 UTC
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 `undef`s. 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]
Re^4: What's going on in array element assignment? by Athanasius (Archbishop) on Aug 11, 2020 at 07:15 UTC
Re^5: What's going on in array element assignment? by zapdos (Sexton) on Aug 12, 2020 at 04:37 UTC
Some notes below your chosen depth have not been shown here
Re^4: What's going on in array element assignment? by zapdos (Sexton) on Aug 11, 2020 at 03:25 UTC


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