Perl references - $$var and \$var notations

For 2023 - 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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))

In Perl, if I write:
$stuff = "Porridge";
then I'm setting up a variable to contain the value "Porridge".

If I write:
$stuffat = \$stuff;
then I'm setting up a variable to contain the ADDRESS OF the variable $stuff.

So if $stuff was in memory at (hex)fe80 ... then $stuffat will be assigned that value 0xfe80. And we can look use it by using an extra $ character :
print $$stuffat;
which literally means "the contents of the contents of stuffat".

What use is this?

Lots ... not only for scalars, but for the addresses of lists too. If I write
$holder = \@biglist;
I have assigned a single scalar variable to point to what is potentially a huge dataset and (for starters) I can easily pass it around to various subs without the need to replicate the data.

Other languages have this concept - "pointer" - too. See & and * in C, and the use of & in PHP.
(written 2009-06-15, updated 2009-06-22)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

P301 - Variables in Perl
  [975] Answering ALL the delegate's Perl questions - (2006-12-09)
  [1581] What is an lvalue? (Perl, C) - (2008-03-18)
  [1946] Variable Types in Perl - (2008-12-15)
  [2374] Lead characters on Perl variable names - (2009-08-24)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [3430] Sigils - the characters on the start of variable names in Perl, Ruby and Fortran - (2011-09-10)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)
  [4608] Introspecion in Perl 6 - (2016-01-02)

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)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [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)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [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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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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