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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
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
P211 - Perl - Hashes
  [240] Conventional restraints removed - (2005-03-09)
  [386] What is a callback? - (2005-07-22)
  [738] (Perl) Callbacks - what are they? - (2006-05-30)
  [930] -> , >= and => in Perl - (2006-11-18)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1705] Environment variables in Perl / use Env - (2008-07-11)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1856] A few of my favourite things - (2008-10-26)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2836] Perl - the duplicate key problem explained, and solutions offered - (2010-06-28)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [3042] Least Common Ancestor - what is it, and a Least Common Ancestor algorithm implemented in Perl - (2010-11-11)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3106] Buckets - (2010-12-26)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [3451] Why would you want to use a Perl hash? - (2011-09-20)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)

P208 - Perl - Lists
  [28] Perl for breakfast - (2004-08-25)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [355] Context in Perl - (2005-06-22)
  [463] Splitting the difference - (2005-10-13)
  [560] The fencepost problem - (2006-01-10)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [773] Breaking bread - (2006-06-22)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2295] The dog is not in trouble - (2009-07-17)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [4609] Mapping an array / list without a loop - how to do it in Perl 6 - (2016-01-03)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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