my %hash = ();
for(my $x = 0; $x <= $#sparse_list; $x++) {
    my $y = $spase_list[$x];
    next if $y eq undef;
    $hash{$x} = $y;
}
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As an aside, you can use exists and delete on array elements but it comes highly discouraged. So don't do it.

The problem is that I need fast *ordered* access, something that a hash really won't help me with.

Ah, now I finally understand what you want. (:

You need a more complex data structure. You can wrap a nice interface around it if you like, but I'll leave that part up to you (or others).

I think you've found a reason to implement a linked list in Perl (normally you don't need such things in Perl). I went with a circular, doubly-linked list for ease of implementation (where "turn 0" is a dummy turn used to find the first and last turns):

my %turn= (
    0=>{Next=>5,Prev=>53},
    5=>{Turn=>"turn 0",Prev=>0,Next=>28},
    28=>{Turn=>"turn 28",Prev=>5,Next=>53},
    53=>{Turn=>"turn 53",Prev=>28,Next=>0},
);
sub pushTurn {
    my( $num, $turn )= @_;
    my $last= $turn{0}{Prev};
    $turn{$num}= {Turn=>$turn,Prev=>$last,Next=>0};
    $turn{0}{Prev}= $num;
    $turn{$last}{Next}= $num;
}
sub deleTurn {
    my( $num )= @_;
    my( $prev, $next )= @{ $turn{$num} }{qw( Prev Next )};
    $turn{$prev}{Next}= $next;
    $turn{$next}{Prev}= $prev;
    delete $turn{$num};
}
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I hope that is close enough to correct that you can get an implementation working. {-:

Use:

my %turn= ( 0=>{Next=>0,Prev=>0} );
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to bootstrap the data structure.
        - tye (but my friends call me "Tye")

Have you considered a linked list in a hash?

Update: Apologies for the redundant post - I hadn't discovered the notes depth setting until a few moments ago, so hadn't noticed tye's post.

Pete

What you are talking about sounds like what a BTree is for. You can get it done in Perl, use DB_File (or the next generation and possibly immature BerkeleyDB) to get it in C.

If you go with the latter two options watch out for the need to set a sort order. You will run into that will the all-Perl solution as well, but there you can modify the module to work that way...