Re: Howto Dynamically Combine Multiple Array in HoA

[Xavier Noria]

On May 4, 2005, at 3:45, Wijaya Edward wrote:
Thanks so much, Xavi.
Sure!
Is that what you wanted?
Yes it works just as I wanted.
Excellent.
This is my final subroutine using your suggestion.
I modified a bit using "reverse" because the order of the concatenated
strings matter.
The last comment in the mail was about this.
sub get_item_set
{
        use Algorithm::ChooseSubsets;
        my %hoa = @_;
        my $iter = Algorithm::ChooseSubsets->new([keys %hoa], 2, 1);
        while (my $keys = $iter->next)
        {
            my $composite_key = join '', reverse @$keys;
This approach is broken because the order in which keys(%hoa) returns  
the keys is random, and this order might be inherited by the iterator  
(or at least you can't assume this is not the case, won't change in  
its next release, etc.), so you are reversing an array with elements  
in an _unknown_ relative order! See the problem? That order in recent  
perls may indeed change even between different runs or perl (for  
security reasons explained in perlsec).

What this means is that sometimes $keys will be ['t1', 't2'], and  
sometimes ['t2', 't1'], from that implementation you can't tell. If  
you need some specific choice, to get the composite key 't1t2'  
instead of 't2t1', the subroutine could explicit some order in the  
arrayref of keys before the iterator is constructed:

    my @sorted_keys = sort { ... } keys %hoa;
    my $iter = Algorithm::ChooseSubsets->new([EMAIL PROTECTED], 2, 1);
Then that'd be also broken because we'd depend on the implementation  
of Algorithm::ChooseSubsets, since from its point of view both ['t1',  
't2'] and ['t2', 't1'] are valid representatives of the subset {'t1',  
't2'} the module can perfectly return any of them. In particular the  
chosen one could change in its next release because that's OK with  
its interface.

So if you can insert the sort criteria down the while block that'd be  
the true robust place to fix the subroutine. We wouldn't sort the  
keys for the iterator, but for the construction of $composite_key:

    my %hoa = @_;
    my $iter = Algorithm::ChooseSubsets->new([keys %hoa], 2, 1);
    while (my $keys = $iter->next)
    {
       # given a representative in $keys generate the one we need
       my $composite_key = join '', sort { ... } @$keys;
Whether a sort is fine or not depends on details of the problem to  
solve though, but you see the issues to take into account.

-- fxn
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