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Autovivification - the magic appearance of variables in Perl

Here's a one line perl program that does nothing but set up a variable ... except that it does a lot!

$p[7]{john}{paul}[9] = 1;
 
__END__
 
A list called @p. Elements 0 to 6 are null, and element 7
is a reference to a hash ...
 
That hash contains a single member, key is "john" and the
values it holds is a reference to another hash ...
 
THAT hash contains a single member with a key of "paul" and
it contains a reference to a list ...
 
And that list has elements 0 to 8 being null, and element
nine containing the value 1.
 
AND THAT WAS ALL DONE AUTOMATICALLY by Perl's Autovivification
capability!!


If you assign to a variable in Perl and it does not already exist, the Perl creates it dynamically - and that implies that it also creates all the other necessary structures as well. My oneliner above took 10 lines to describe in plain English!

This is both powerful and dangerous. Something as simple as

$phone[1225708225] = 1;

will give you "out of memory" as a list of over one billion elements is calledup. But do remember that

$phone{1225708225} = 1;

will create a hash with just one member, and THAT is what you should use for data with sparsely distributed numeric keys
(written 2008-01-21)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P217 - Perl - More than Simple Lists and Hashes!
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [3406] Not multidimentional arrays - but lists of lists. Much more flexible. Perl! - (2011-08-26)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [293] Course follow-ups - (2005-04-27)
  [43] Hash of lists in Perl - (2004-09-09)


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