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.