perl adds dummy elements to array after inquiry

Question

As the title says, perl adds dummy elements to arrays after inquiries to not existing elements. Array size grows after the inquiry. Illustration to the behaviour:

    my $rarr;
    $rarr->[0][0] = 'S';
    $rarr->[0][1] = 'MD';
    $rarr->[1][0] = 'S';
    $rarr->[1][1] = 'PRP';

    my $crulesref;
    $crulesref->[0]  = $rarr;

     check_rule('aa', 0);
     if($rarr->[3][0] == 'M'){  # just check a not existing element
        print "m\n";   
     }

     check_rule('bb', 0);
     if($rarr->[5][0] == 'M'){  # again: just check a not existing element
        print "m\n";
     }
     check_rule('cc', 0);


     sub check_rule($$)
     {
         my ($strg,$ix) = @_;
         my $aref = $crulesref->[$ix];
         my $rule_size = @$aref;
         {print "-----$strg aref:$aref rs:$rule_size aref:'@$aref'\n";
           for(my $t1 = 0; $t1 <$rule_size; $t1++){
             print "t1:$t1 0:$aref->[$t1][0] 1:$aref->[$t1][1]\n";
           }
         }
       }

The result of the run is:

    en@en-desktop ~/dtest/perl/forditas/utf8_v1/forditas/test1 $ perl v15.pl
    -----aa aref:ARRAY(0x90ed8c8) rs:2 aref:'ARRAY(0x9106cac) ARRAY(0x9106d24)'
    t1:0 0:S 1:MD
    t1:1 0:S 1:PRP
    m                     <-------------- finds the non existing
    -----bb aref:ARRAY(0x90ed8c8) rs:4 aref:'ARRAY(0x9106cac) ARRAY(0x9106d24)          ARRAY(0x9107508)'
    t1:0 0:S 1:MD
    t1:1 0:S 1:PRP
    t1:2 0: 1:               <-- undesired dummy due to inquiry
    t1:3 0: 1:               <-- undesired dummy due to inquiry
    m                      <-------------- finds the non existing
    -----cc aref:ARRAY(0x90ed8c8) rs:6 aref:'ARRAY(0x9106cac) ARRAY(0x9106d24) ARRAY(0x9107904) ARRAY(0x9107508)  ARRAY(0x910e860)'
    t1:0 0:S 1:MD
    t1:1 0:S 1:PRP
    t1:2 0: 1:               <-- undesired dummy due to inquiry
    t1:3 0: 1:               <-- undesired dummy due to inquiry
    t1:4 0: 1:               <-- undesired dummy due to inquiry
    t1:5 0: 1:               <-- undesired dummy due to inquiry

Is there no other way to avoid this than to ask before each inquiry, if the inquired element exists? I try to increase speed, and these inquiries slow the code down, and make it less easy to read.

Thanks in advance for useful hints.

Answer 1

This is autovivification that you are seeing. If you access the memory of $ref->[3][0] even with just a check:

if ($ref->[3][0] eq 'M' )

Then first $ref->[3] must exist before its element number zero can be checked, so it is created via autovivification. You need to first check if $ref->[3] exists or is defined to avoid creating it.

if (defined($ref->[3]) && $ref->[3][0] eq 'M')

Also, you should always use:

use strict;
use warnings;

Then you would see the warnings

Argument "M" isn't numeric in numeric eq (==) at ...
Use of uninitialized value in numeric eq (==) at ...

The if-clause gives a false positive here because the string 'M' is converted to a number (0) because of the context imposed by the numeric equality operator ==. The LHS value is undef, which is also converted to a number (0), which is why the expression evaluates to true.