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$ is atomic and % and @ are molecular - Perl

"When do I use a $, when do I use an @, and when do I use a % ?" That is a frequently asked question on a Perl course, where a delegate had dabbled with a bit of Perl ahead of time.

Methane MoleculeYou use @ if you're referring to the whole of a list - in other words a complete collection. If you're a chemist, you may refer to methane as CH4 - and that's a molecule - a collection.

And you use $ if you're referring to a single scalar value rather than a colelction as a whole. It might be a quite separate scalar variable, or it might be an individual member of a list in which cas you'll put its index number (position) in the list in square brackets. The $ is a single value - an atom to the chemist.

Here's an example to show what I mean

  # Whole list with @
  @menu = ("Coffee","Muffin","Plate","Toast","Tea");
  print "@menu\n";
  
  # Individual member with $
  $menu[2]="Croissant";
  print "$menu[1]\n";
  
  # Whole list again - with changed individual member
  print "@menu\n";


And when we run that ...

  munchkin:ap1 grahamellis$ perl lz
  Coffee Muffin Plate Toast Tea
  Muffin
  Coffee Muffin Croissant Toast Tea
  munchkin:ap1 grahamellis$


There's a sample showing various examples [here] from yesterday's Perl course ... and there are older examples (which are included and documented in our training notes that accompany the courses [here] and [here].





So - what about the % character? That's used to access a hash. A list (which we saw above) is a collection of sequentially numbered scalars, from 0 up, whereas the members of a hash are (typically) named rather than numbered, and aren't in any particuar ordered sequence.

Here's a code sample like the one above - this time using a hash:

  # Whole hash with %
  %menu = (Bob => "Coffee",Brenda => "Muffin",Thomas => "Plate",
       Felicity => "Toast",Sasha => "Tea");
  print %menu,"\n";
  
  # Individual member with $
  $menu{Thomas}="Croissant";
  print "$menu{Brenda}\n";
  
  # Whole hash again - with changed individual member
  print %menu,"\n";


And running that:

  munchkin:ap1 grahamellis$ perl hz
  SashaTeaFelicityToastBobCoffeeThomasPlateBrendaMuffin
  Muffin
  SashaTeaFelicityToastBobCoffeeThomasCroissantBrendaMuffin
  munchkin:ap1 grahamellis$


With a hash, you can't simply loop through a series of index numbers to reference items by their position number - you'll normall use the keys function. There's an example here - again, we cover that in detail on our Perl courses.
(written 2011-08-20)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P208 - Perl - Lists
  [4609] Mapping an array / list without a loop - how to do it in Perl 6 - (2016-01-03)
  [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)
  [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)
  [968] Perl - a list or a hash? - (2006-12-06)
  [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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$ is atomic and % and @ are molecular - Perl
From fish, loaves and apples to money, plastic cards and BACS (Perl references explained)
Perl - making best use of the flexibility, but also using good coding standards
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