in reply to Multiple Levels of Array Dereferencing Have me Stumped

you wrote: 
# umm -- uh.... well, the @sign implies
# we have an array, but how is it 
# different from the first array we
# dereferenced?

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 It's a copy!
see this example: 
my $ref1 = ['a','b','c'];
my $ref2 = ['d','e','f'];

my $refref = [$ref1, $ref2];

myfunc($refref);
print $ref1->[0];

sub myfunc {
        my (@list) = @{[@{$_[0]}]};
        print $list[0][0];
        $list[0] = ['1','2','3'];
}

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it prints 'aa' and not 'a1' because @{[(list of refs)]} has built a copy of 'list of refs' protecting $ref1 from being altered by the assignment inside the funcion
my 2 cents...

Replies are listed 'Best First'.
RE (tilly) 2: Multiple Levels of Array Dereferencing Have me Stumped
by tilly (Archbishop) on Nov 05, 2000 at 23:11 UTC
    Yes, but is it a useful copy? The question was about an extra reference-dereference pair that makes a copy of a copy. What do you get for that work?

    Just about nothing. There is no reason that I can see to prefer: 

    sub foo {
  my @foo = @{[@{$_[0]}]};
  # etc
}

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    over 
    sub foo {
  my @foo = @{$_[0]};
  # etc
}

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    In fact try this: 
    my $bar = ["hello", "world"];
foo($bar);
print "$bar->[0]\n";

sub foo {
  # Make my copy.
  my @foo = @{$_[0]};
  # Try to mess up the first element.
  $foo[0] =~ s/h/H/;
}

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    See? You get a private copy already.
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[d/l]
[select]
      I was trying to explain "what" that code does and not "why"
      however you'r right: my (@list) = @{[@{$_[0]}]}; will just build an extra (useless) copy, i didn't verified it..
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[d/l]
        It looks to me like metaperl had already figured that out and the confusion was the belief that the extra copy had to do something useful.

        Which it doesn't.

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RE: Re: Multiple Levels of Array Dereferencing Have me Stumped
by Fastolfe (Vicar) on Nov 06, 2000 at 00:41 UTC
    But wait a moment. Isn't @array = @{$arrayref} a copy also? Modifying $array[0] should have no effect upon $arrayref->[0]. So the extra level of de-referencing is puzzling.

    Setting one array to another copies the values, and by de-referencing the first time, you're building an array on the right-hand-side from which to copy.

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[d/l]
[select]
RE: Re: Multiple Levels of Array Dereferencing Have me Stumped
by mdillon (Priest) on Nov 05, 2000 at 22:29 UTC
    i'd say the having the most correct answer is worth more than 2 cents.
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(ar0n) RE (2): Multiple Levels of Array Dereferencing Have me Stumped
by ar0n (Priest) on Nov 05, 2000 at 22:49 UTC
    Occam's razor! It's great!

    [ar0n]

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