Taking the dog out for a walk? It's much easier to use a lead than to carry the whole dog.

And ... in a similar vein ... when you're passing some data into a named block of code (a.k.a. sub, subroutine, function, procedure, command, method, macro) when programming, it's much more efficient to pass in a pointeri (a.k.a. an address or a reference)

When you pass in a pointer, you're saving yourself having to duplicate the data. That can be a serious saving if you're working with large or huge data sets. And you're also providing a powerful (but dangerous if misused) capability of altering the original data within the code that you've called.

On yesterday's Perl course, I wrote a fresh example that compares copying a list into a Perl sub, and passing in the address. The difference is just a \ !


Copying into the sub:
  addon("Ibiza",@places);

Passing a reference into the sub:
  anotheradd("Magaluf",\@places);

Full source code of the demo [here], including the source of the subs which must (of course) be coded to accept the data type that's being passed to them.

---

Perl's references are also an excellent way of setting up collections of collections - a very flexible and space-saving alternative to the multidimensional arrays of other languages. New example of those - [here].
(written 2011-09-17, updated 2013-01-01)

 P217 - Perl - More than Simple Lists and Hashes!
  [43] Hash of lists in Perl - (2004-09-09)
  [293] Course follow-ups - (2005-04-27)
  [1514] Autovivification - the magic appearance of variables in Perl - (2008-01-21)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3406] Not multidimentional arrays - but lists of lists. Much more flexible. Perl! - (2011-08-26)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)

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Checking all the systems on a subnet, using Expect and Tk
Needle in a haystack - finding the web server overload
Awk v Perl
Perl and CGI - simple form, and monitoring script.
Take the dog on a lead - do not carry her. Perl references.
Getting more log information from the Apache http web server
A demonstration of how many Python facilities work together
Pressing ^C in a Python program. Also Progress Bar.
Research is exciting. But should routine be automated?
Python for loops - applying a temporary second name to the same object