Exercises, examples and other material relating to training module P304. This topic is presented on public course Perl for Larger Projects
Perl variables can be given nonstandard characteristics, such as persistence so that they won't be lost when you exit from a program and then start it again. There are a number of exciting things you can do with this facility, known as tieing, but you also need to be careful that the code you write is easy to understand and maintain too.
|Articles and tips on this subject||updated|
|3409||When variables behave differently - Tie in Perl|
Deep in the bowels of any programming language is code to store information into a variable, and get information back from that variable. And in a language that's dynamic in its memory allocation, there will also be code to construct and destroy variables.
Does that sound a bit "object oriented". ...
|3007||Setting up a matrix of data (2D array) for processing in your program|
When you're reading and processing data, it often comes in the form of a series of records, with each record being split into a series of fields, and you'll often want to be going through the data several times, looking at different rows and colums, sorting them, comparing them, and so on. If the amount ...
|2243||Changing a variable behaviour in Perl - tieing|
What 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 ...
|2379||Making variables persistant, pretending a database is a variable and other Perl tricks|
Have a look at this Perl program:
$counter = $counter + 1;
print ("This is access no. $counter\n");
Apart from the rather curious module loaded at the top, this seems to take an undefined variable, set it to one, and print it out. What a - err - pointless (!) ...
Examples from our training material
|ac2|| Using a tie class to force lower case|
|ac3|| Using a tie class to force lower case|
|acm2|| tieing a text file to a hash|
|acmagic|| Persistant hash - NOT lost when the program exits|
|acnlook|| Tieing a database file to a hash|
|acnset1|| Crreating a database file through tie|
|acnset2|| Tying a text file to a hash|
|ah|| Tieing a variable to change its behaviour|
|alias_list|| Accessing Unix's aliases database directly|
|allcodes|| tied hash, unique key solution|
|aphash.pm|| tie class definition - an appending hash|
|bowtie|| Using a variable tied to a file|
|codes|| location lookup|
|codes2|| Place lookup without tie - variables are cas sensitive|
|flash.pm|| Appending hash tie class definition|
|fyle.pm|| links a variable to a file|
|game|| Appliaction that uses a scalar as a stack|
|lower.pm|| Tie class to force all strings to lower case|
|persist|| Persistant variable via a tie class|
|pwl2|| Looking up user accounts, case insensitive|
|pwlook|| Looking up users, case sensitive|
|stack.pm|| Tie class that hides a stack under a scalar|
|textfile.pm|| tie class that treats a file as a scalar|
|thash.pm|| Tie class to access a disc file as a hash, with caching|
PicturesTrying out the subject taught in the last lecture
Some modules are available for download
as a sample of our material or under an Open Training Notes License
for free download from [here]
Topics covered in this module
Tieing to scalars, hashes and lists.
Tieing to a scalar.
Tieing to a hash.
Tieing to make data more permanent.
Tieing a text file to a scalar.
Tieing a text file to a hash.
Tieing to database files.
Accessing system databases.
Tieing File Handles.
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