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Re: Dereference anonymous AoA

by Anonymous Monk
on Aug 08, 2014 at 19:41 UTC ( #1096807=note: print w/replies, xml ) Need Help??


in reply to Dereference anonymous AoA

Foreach loop provides list context, so foreach my $i ( @{ $aref } ) explodes into its elements

foreach my $i( ['kept', 82], ['notkept', 1], ['repaired', 3] ) { ... # so $aref->[$i][0] # looks like $areg->[ ['kept', 82] ][0] #
Here the first lines out error output:
$ perl buggy_array.pl 2> out.txt (it hangs here, press Ctrl-C to stop) $ head -6 out.txt Use of reference "ARRAY(0x1cfb148)" as array index at ary.pl line 21. Use of uninitialized value in concatenation (.) or string at ary.pl li +ne 21. Use of reference "ARRAY(0x1d0fad8)" as array index at ary.pl line 21. Use of uninitialized value in concatenation (.) or string at ary.pl li +ne 21. Use of reference "ARRAY(0x1d0fbf8)" as array index at ary.pl line 21. Use of uninitialized value in concatenation (.) or string at ary.pl li +ne 21.
Now, why does it keep looping?
perl -E 'my $aref=[undef]; say 0 + $aref' 19468616
19468616 is the memory address of $aref here. Which means
$areg->[ ['kept', 82] ][0] # becomes something like # $areg->[ 9999999 ][0] # (or whatever the memory address of that array is...)
When you do stuff like $aref->[999][0] and element 999 of $aref doesn't exist, Perl will extend $aref. It will fill it with undef, up to element 999.
perl -E 'my $aref=[undef]; $aref->[1000][0]; say scalar @{ $aref }' 1001
They call it 'autovivification' (sp?). (but if you do $aref->[999][500], Perl will not create the rightmost array - the one with 500 elements)

So, now your $aref has tons of of undefs. So the loop keeps looping and issuing warnings.

So yeah, all in all, that was a pretty interesting question, I must say.

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