In Perl, how to use 'defined' function on elements of two-dimensional array?

Question

I am trying to check if an element is defined, using defined function in Perl.

Code :

$mylist[0][0]="wqeqwe";
$mylist[0][1]="afasf";
$mylist[1][0]="lkkjh";

print scalar(@mylist), "\n";

if (defined($mylist[2][0])){print "TRUE\n";}

print scalar(@mylist), "\n";

Output

2
3

Before using defined function, there were two elements in first dimension of @myarray. After using defined function, the number of elements increase to 3.

How to use defined function with out adding new elements ?

Answer 1

First check that the first-level reference exists.

if ( defined($mylist[2]) && defined($mylist[2][0]) ) {
    print "TRUE\n";
}

What you've encountered is called autovivification: under some circumstances, Perl creates complex data structures when you use them as if they already existed.

Answer 2

It's interesting to note that there's a non-core pragma called autovivification, and that if you run your code under no autovivification; your problem will go away.

Answer 3

When you refer to $mylist[2][0], perl's autovivification creates the array element $mylist[2].

To prevent this, you can check this element first:

if ( (defined $mylist[2]) && (defined $mylist[2][0]) )

Answer 4

defined($mylist[2][0])

is equivalent to

defined($mylist[2]->[0])

which is short for

defined( ( $mylist[2] //= [] )->[0])

due to autovivification. You can disable autovivification using the autovivification pragma.

no autovivificatoin;
if (defined($mylist[2][0]))

Or you can avoid evaluating code that would trigger it.

if (defined($mylist[2]) && defined($mylist[2][0]))

Answer 5

Actually because it's autovivification, you can check it easily with Data::Dumper, before and after using defined.

use Data::Dumper;
my @mylist;

$mylist[0][0]="wqeqwe";
$mylist[0][1]="afasf";
$mylist[1][0]="lkkjh";

print Dumper(@mylist);

Output before

$VAR1 = ['wqeqwe', 'afasf'];
$VAR2 = [ 'lkkjh'];


print Dumper(@mylist);

Output after

$VAR1 = [ 'wqeqwe','afasf' ];
$VAR2 = ['lkkjh'];
$VAR3 = [];