Beefy Boxes and Bandwidth Generously Provided by pair Networks
There's more than one way to do things
 
PerlMonks  

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

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.


In reply to Re: Dereference anonymous AoA by Anonymous Monk
in thread Dereference anonymous AoA by neilwatson

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or or How to display code and escape characters are good places to start.
Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others making s'mores by the fire in the courtyard of the Monastery: (6)
As of 2022-08-11 18:01 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    No recent polls found

    Notices?