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Perl - a list or a hash?

You can hold multiple scalars in either a list or a hash in Perl. A list (signaled by an @ character or [..] around the subscript) is ordered - i.e. the elements are numbered and the order is significant. In contrast, a hash (signalled by a % character or {..} around the subscript) is unordered; the elements are named and the order isn't easily predicted.

Why would anyone want to use a hash if it's not in order? Well - if you want to look things up based on a key it's very easy and very efficient. And if you've keyed data and use lists, it's quite awkward to write the code to keep them in step. Here's an example:

# Using a list - works but bulky

@onco = ("Nathan","Steve","Andrew");
@from = ("London","Romford","Cardiff");

print "Where is ... from ? ";
chop ($name = <STDIN>);

for ($k=0; $k<@onco; $k++) {
  if ($onco[$k] eq $name) {
    print "$name is from $from[$k]\n";
  }
}

# Using a hash - an unordered collection

%pep_tab = ("Nathan" => "Brighton",
  "Steve" => "Walthamstow",
  "Andrew" => "London");

print "Where is ... from ? ";
chop ($name = <STDIN>);

print "$name was from $pep_tab{$name}\n";


It turns out that hashes have many, many other uses too - their very quick look up algorithm is great for anything from checking for a word in a dictionary through to analysing all the client computers registed in your web access log.
(written 2006-12-06, updated 2006-12-07)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P208 - Perl - Lists
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2295] The dog is not in trouble - (2009-07-17)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [773] Breaking bread - (2006-06-22)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [560] The fencepost problem - (2006-01-10)
  [463] Splitting the difference - (2005-10-13)
  [355] Context in Perl - (2005-06-22)
  [240] Conventional restraints removed - (2005-03-09)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [28] Perl for breakfast - (2004-08-25)

P211 - Perl - Hashes
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3451] Why would you want to use a Perl hash? - (2011-09-20)
  [3106] Buckets - (2010-12-26)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3042] Least Common Ancestor - what is it, and a Least Common Ancestor algorithm implemented in Perl - (2010-11-11)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2836] Perl - the duplicate key problem explained, and solutions offered - (2010-06-28)
  [1856] A few of my favourite things - (2008-10-26)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1705] Environment variables in Perl / use Env - (2008-07-11)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [930] -> , >= and => in Perl - (2006-11-18)
  [738] (Perl) Callbacks - what are they? - (2006-05-30)
  [386] What is a callback? - (2005-07-22)


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