CODEWITHSUNDEEP
arraysperlmultidimensional-array
kiasectomondo
kiasectomondo

Reputation: 45

perl - trouble understanding array of arrays

I am trying to make an array of arrays and then reference them.

I make somethign like:

sub foobar
{
 my @array;
 my $i;
 for ($i = 0; $i < 1000; $i=$i+1)
 {
  my @row;
  $row[0] = $i;
  $row[1] = foo($bar);
  push @array , [@row];
 }
 return \@array;
}

I can get to the values via:

$array->[x]->[y];

However I don't understand why the second -> is needed. $array->[x] I understand because $array is a reference. But isn't $array->[x] meant to be an array? Why doesn't this work:

my @notarray = $array->[x];

What exactally is not array filled with now? Because it certainly doesn't seem to be an array containing $i , foo($bar)

How would $array->[x]->[y] be different for a reference to an array of references to arrays?

Upvotes: 1

Views: 784

Answers (2)

Platinum Azure
Platinum Azure

Reputation: 46183

The second -> isn't needed, actually.

Here's the deal: All Perl array values and hash values must be scalars. That means either a string, number, or array/hash reference (and not a plain old array or hash).

So the first -> operator dereferences the array and gets at the x'th row. In there is-- not an array, but an array reference. So in order to get to the data in there, you'd theoretically need another -> operator.

But get this. Perl is smart: It knows that after one array or hash access, if another access happens, the only way this is possible is through an array/hash reference (because your first array/hash access MUST return a scalar)! So you don't need the second arrow after all.

See perldata for more details.

Upvotes: 4

Ted Hopp
Ted Hopp

Reputation: 234795

When you

push @array, [@row];

you are pushing a reference to an array. This is necessary because of Perl's rule that arrays are flattened. So $array->[x] is a reference to the row array, not the row array itself. However, between subscripts, the arrow is optional. So $array->[x]->[y] is exactly the same as $array->[x][y] (which is exactly the same as ${$array}[x][y], etc.)

This is all explained in the Perl reference tutorial

Upvotes: 2

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