G'day chaoticset,

I'm assuming that we have lists that look like this:

@oldstock = ("widgets:10:5",
             "wodgets:4:2",
             "spanners:5:3");

@newstock = ("screwdrivers:8:3",
             "hammers:5:1",
             "widgets:3:1");
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And we want to end up with a list like this:

@totalstock = ("widgets:13:6",  # Both widget entries joined
               "wodgets:4:2",
               "spanners:5:3",
               "screwdrivers:8:3",
               "hammers:5:1");
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I'm also assuming that the lists are un-ordered, and the ordering of the final list does not matter.

This is a perfect opportunity to use hashes. We have an obvious key (the name), and values (stock and shown). Given that all we really want to do is join the lists together, we should just be able to iterate through them, use some hashes to build our results, and them print them. Here's an example using the arrays above:

my %stock = ();
my %shown = ();

# Build our records

foreach my $record (@newstock @oldstock) {
    my ($name, $stock, $shown) = split(/:/$record);
    $stock{$name} += $stock;
    $shown{$name} += $shown;
}

# Print the result.

foreach my $name (keys %stock) {
    print join(":",$name,$stock{name},$shown{name}),"\n";
}
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You could do the same thing using a single hash and references, but I believe the above is more simple to understand.

Hope that the above helps.

Cheers,
Paul

Update: Updated to not check for the existance of entries in the hashes, but just += them anyway. I was doing an explicit test with exists() in order to avoid warnings, but as it happens using += with an undefined lvalue doesn't produce a warning anyway. Thanks to cLive ;-) for pointing this out.