Question

I once read the following Perl code involving iterations.

for my $j (0 .. $#{$dat[$Row]})
{
    $vectors{ $dat[$Row][$j] } = $j;
}

What does

$vectors{ $dat[$Row][$j] } 

stand for?
Is that equivalent to $vectors->$dat[$Row][$j] ?

Answer 1

what does $vectors{ $dat[$Row][$j] } stand for?

$dat[$Row] is a reference to an array. $dat[$Row][$j] is apparently an element in that array. Whatever value is contained in it, becomes a hash key in %vectors, which gets the value $j.

Is that equivalent $vectors->$dat[$Row][$j]

No, that would be referring to the variable $vectors, not %vectors.

A more readable way to write this might be:

my $aref = $dat[$Row];   
for my $index (keys @$aref) {
    my $key = $aref->[$index];
    $vectors{$key} = $index;
}

Which also exemplifies the use of ->, to dereference a reference.