Making variables persistant, pretending a database is a variable and other Perl tricks

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Making variables persistant, pretending a database is a variable and other Perl tricks

Have a look at this Perl program:

use fyle;

tie $counter,"fyle";

 

$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 (!) program. But:

Dorothy-2:pl grahamellis$ perl bowtie

This is access no. 11

Dorothy-2:pl grahamellis$ perl bowtie

This is access no. 12

Dorothy-2:pl grahamellis$ perl bowtie

This is access no. 13

Dorothy-2:pl grahamellis$ perl bowtie

This is access no. 14

Dorothy-2:pl grahamellis$

What is happening?

Deep within Perl, there are a very limited number of operations that can be performed with a scalar - it can be created and destroyed, it can be read and written ... and that's about it. And these operations can be redefined by a class, and then applied to individual variables via a tie command ... which is what I have done here.

So in my example, the variable is persistent. Inside the class, it's actually stored in a data file that is read each time the variable is read, and overwritten each time a new values is saved to the variable. So it lasts from one run of the program to the next.

The source code shown above is also available here, and the definition of tieing a variable to a filecontents is here.

You can do a lot more with tieing. We have an example of connecting to a flat file database (NDBM) here (that calls up a standard tie class shipped with Perl). There's an example that forces strings in a variable to always be treated as lower case here and here. Finallly, a tie class which maps a file to a hash here.

Another new example / Perl trick ... see here for a short example / explanation of binary data handling in Perl
(written 2009-08-27, updated 2009-08-28) 2c1a

Associated topics are indexed under

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)
  [2243] Changing a variable behaviour in Perl - tieing - (2009-06-16)

P212 - Perl - More on Character Strings
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [608] Don't expose your regular expressions - (2006-02-15)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [583] Remember to process blank lines - (2006-01-31)
  [453] Commenting Perl regular expressions - (2005-09-30)

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Some other Articles

Lua Regular Expressions
Giving up on user input - keyboard timeout in Perl
Checking the database connection manually
Object Oriented programming - a practical design example
Making variables persistant, pretending a database is a variable and other Perl tricks
Handling XML in Perl - introduction and early examples
Wiltshire / Melksham Weddings - guest accommodation
Long job - progress bar techniques (Perl)
Designing your data structures for a robust Perl application
Lead characters on Perl variable names

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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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