One of the vital topics on all our programming courses is that of variable scope. Variable Scope may be defined as the area of a program in which a variable is visible, and how long that variable is accessible for.

Why do I describe variable scope as a vital subject when you can write simple programs in almost any of the languages we teach without any appreciation of the issue? Because it becomes vital as your code grows into a series of blocks - sometimes named (functions in PHP, procs in Tcl, def in Python, subs in Perl, methods in Java) and sometimes not (for example { to } blocks in Perl, inset blocks in Python). As a new programmer, you might think that all variables should be visible everywhere (this is often know as "default global"), but that's a bad default - the only language that has this default is Perl and that's for historic reasons. It will change at Perl 6!

Why is "default global" a bad default? Because it means that when you combine code from two blocks / files / sources, each must have variables who's names don't conflict with and other name in any other module and in practice that's almost impossible to ensure. Looking at a real-life example, in our household "Charlie" refers to the cat, but in the Royal Family "Charlie" might be used to refer to the Prince of Wales. Let's say that the Royal family comes to visit us (yeah, improbable I know!), and someone says "come here Charlie" .... are they going to get a cat or a prince?
(written 2004-10-22 07:32:43)

 H105 - PHP - Functions
T207 - Tcl/Tk - Procedures and Variable Scope
Y105 - Python - Functions, Modules and Packages
P209 - Subroutines in Perl

Back to Five red flags

Previous and next or Horse's mouth home

Forward to What makes a professional photographer?

Life is frail
Should we call it 'Geekmas'?
No more 'Error 404' pages. Something better.
What makes a professional photographer?
Variable Scope
Five red flags
Special change
Case Sensitive?
Hard work, Hard sell
On line every 24 hours
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 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).