Question

I have seen one easy program. Well, the hole program i have understand except one thing and that is how Hash is working here :

Program ->Extracting Unique Elements from a List

@list = (20,30,40,60,40,20,30,2);
@uniq = ();
%seen = ();
foreach $item (@list) {
    unless ($seen{$item}) 
    {
        # if we get here, we have not seen it before
        push(@uniq, $item);
        $seen{$item}++;

    }
    print %seen;
    print"\n";
}

My question is how hash is comparing from its exiting value to current value i.e how it is checking weather that value is already there or not .If I m printing the %seen i am getting some value.how those values are coming ?

Answer 1

If it makes it clearer for your, change

if (!$seen{$item}) { $seen{$item}++; ... }

to

if (!exists($seen{$item})) { $seen{$item} = 1; ... }

The first time you encounter a particular item, it doesn’t exist as a key in the hash, so the if is entered. The body of the if creates a key in the hash equal to the item.

The second (and third and …) time you encounter a particular item, it exists as a key in the hash, so the if is not entered.

---

By the way,

if (!$seen{$item}) { $seen{$item}++; ... }

can be shortened to

if (!$seen{$item}++) { ... }

And

my @uniq;
for my $item (@list) {
    push @uniq, $item if ...;
}

can be shortened to

my @uniq = grep ..., @list;

So the whole thing can be written as

my @list = (20,30,40,60,40,20,30,2);
my %seen;
my @uniq = grep !$seen{$_}++, @list;