Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Retiring, March 2020 - sorry, you have missed our final public course.
The Coronavirus situation has lead us to suspend public training - which was on the cards anyway, with no plans to resume

Please ask about private 'maintenance' training for Python, Tcl, Perl, PHP, Lua, etc
Happily continuing private consultancy / programming work
 
Changing a variable behaviour in Perl - tieing

On a course at Well House ManorWhat can you do with a scalar variable? When you reduce it to lowest level programming principles, not a lot ... you can create it, destroy it, save a value into it, and read a value back from it. That's about it, when you think of it!

In Perl, the low level memory accessors within the language go through just a handful of routines (it's really an object interface), which you can override for individual variables if you like. What does this mean? It means you can change the characteristics of a variable if you like. Look at this program:

use propercase;
tie $yourname,"propercase";
print "Enter your name ";
$yourname = <STDIN>;
print "Welcome, $yourname";


Good - but because of my propercase class it will force the name to be properly cased:

earth-wind-and-fire:~/jun09 grahamellis$ perl ppc
Enter your name graHam
Welcome, Graham
earth-wind-and-fire:~/jun09 grahamellis$


Clever, isn't it? ... Here is the module that defines "propercase"

package propercase;
sub TIESCALAR {
  my ($class,$value) = @_;
  bless \$value,$class; }
sub STORE {
  my ($inst,$value) = @_;
  $$inst = ucfirst(lc($value)); }
sub FETCH {
  my ($inst) = @_;
  return $$inst; }
1;


Other examples of a tied scalar are for a variable which automatically has spaces trimmed off the start and end, a stack (where each time to save a value it is actually pushed onto a list, and each time you call it back, the next value is returned) and a variable that persists between runs of your program, as tieing links tha variable to a file on the disc.

Tieing is also possible to lists and hashes - in the case of a hash it can provide a very useful interface indeed to a database table - you can insert / replace / select / update database records with simple assignment statements. The methods you need to implement to use a tied hash are TIEHASH (the constructor), STORE, FETCH, EXISTS, DELETE, CLEAR, FIRSTKEY and NEXTKEY. There are examples in our Tieing in Perl resource module, which is covered on our Perl for Larger Projects course.


Illustration - delegates on a Perl course at Well House Manor
(written 2009-06-16, updated 2010-06-23)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P304 - Perl - Tieing
  [3409] When variables behave differently - Tie in Perl - (2011-08-28)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)


Back to
So what is this thing called Perl that I keep harping on about?
Previous and next
or
Horse's mouth home
Forward to
What should a web site cost you?
Some other Articles
A day in the life of a hotelier
What difference does using the XHTML standard really make?
Client side (Applet) and Server side (Servlet) Graphics in Java
What should a web site cost you?
Changing a variable behaviour in Perl - tieing
So what is this thing called Perl that I keep harping on about?
Perl references - $$var and \$var notations
How do I query a database (MySQL)?
Cornerstone Cafe, Melksham
Handling nasty characters - Perl, PHP, Python, Tcl, Lua
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


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.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2020: 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/2243_Cha ... ieing.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat