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Variable Scope

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, updated 2008-05-11)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
H105 - PHP - Functions
  [3026] Coding efficiency - do not repeat yourself! - (2010-11-02)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [2737] Improving your function calls (APIs) - General and PHP - (2010-04-24)
  [2682] Adding extensions to PHP Open Source applications - callbacks - (2010-03-17)
  [2630] Static variables and ampersands in PHP - (2010-02-10)
  [2488] A variable number of arguments in a PHP function - (2009-11-02)
  [1784] Global - Tcl, PHP, Python - (2008-09-03)
  [1380] Static variables in PHP - (2007-10-05)
  [1357] Clean my plate, but keep my wine bottle. (PHP; Static) - (2007-09-18)
  [1267] is there a lookup function in php? - (2007-07-15)
  [1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1021] PHP - static declaration - (2007-01-04)
  [936] Global, Superglobal, Session variables - scope and persistance in PHP - (2006-11-21)
  [866] A lazy programmer is a good programmer - (2006-09-15)
  [775] Do not duplicate your code - (2006-06-23)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [409] Functions and commands with dangerous names - (2005-08-11)
  [340] Code and code maintainance efficiency - (2005-06-08)
  [339] Passing information into and out of PHP functions - (2005-06-07)
  [308] Call by name v call by value - (2005-05-11)
  [223] There is a function in PHP to do that - (2005-02-21)

J706 - Java - Objects and Classes
  [4422] Objects - from physical to virtual or abstract - Java - (2015-02-10)
  [4413] Binomial Coefficient (Pascal Triangle) objects in Java - (2015-02-03)
  [2651] Calculation within objects - early, last minute, or cached? - (2010-02-26)
  [2616] Defining a static method - Java, Python and Ruby - (2010-02-01)
  [2422] Looking inside Java classes - javap and javadoc - (2009-09-25)
  [2169] When should I use OO techniques? - (2009-05-11)
  [1925] Introduction to Object Oriented Programming - (2008-12-06)
  [1906] Long, Longer, Longest in Java - (2008-11-25)
  [1500] First Class Java. First step and moving forward. - (2008-01-10)
  [1296] An example of Java Inheritance from scratch - (2007-08-08)
  [477] Class, static and unbound variables - (2005-10-25)

P209 - Subroutines in Perl
  [3833] Learning to use existing classes in Perl - (2012-08-10)
  [3574] Perl functions such as chop change their input parameters - (2012-01-10)
  [3066] Separating groups of variables into namespaces - (2010-11-24)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2069] Efficient calls to subs in Perl - avoid duplication, gain speed - (2009-03-07)
  [1921] Romeo and Julie - (2008-12-04)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1850] Daisy the Cow and a Pint of Ginger Beer - (2008-10-21)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1782] Calling procs in Tcl and how it compares to Perl - (2008-09-02)
  [969] Perl - $_ and @_ - (2006-12-07)
  [588] Changing @INC - where Perl loads its modules - (2006-02-02)
  [531] Packages in packages in Perl - (2005-12-16)
  [357] Where do Perl modules load from - (2005-06-24)

T207 - Tcl/Tk - Procedures and Variable Scope
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)
  [3629] Sharing lots of values in Tcl without having lots of global declarations - (2012-02-28)
  [3414] Passing back multiple results in Tcl - upvar and uplevel - (2011-09-01)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [2520] Global and Enable - two misused words! - (2009-11-30)
  [2476] Tcl - uplevel to run code at calling level - (2009-10-24)
  [2466] Tcl - passing arrays and strings in and back out of procs - (2009-10-22)
  [1404] Tcl - global, upvar and uplevel. - (2007-10-24)
  [122] Passing arrays to procs in Tcl - (2004-11-18)

Y105 - Python - Functions, Modules and Packages
  [4724] From and Import in Python - where is the module loaded from? - (2016-11-06)
  [4722] Embedding more complex code into a named block - (2016-11-04)
  [4719] Nesting decorators - (2016-11-02)
  [4662] Recursion in Python - the classic example - (2016-03-07)
  [4645] What are callbacks? Why use them? An example in Python - (2016-02-11)
  [4448] What is the difference between a function and a method? - (2015-03-04)
  [4441] Reading command line parameters in Python - (2015-02-23)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4407] Python - even named code blocks are objects - (2015-01-28)
  [4361] Multiple yields and no loops in a Python generator? - (2014-12-22)
  [4212] Python functions - an introduction to how they work - (2013-11-16)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [3945] vargs in Python - how to call a method with unknown number of parameters - (2012-12-06)
  [3931] Optional positional and named parameters in Python - (2012-11-23)
  [3885] Default local - a good choice by the author of Python - (2012-10-08)
  [3852] Static variables in Python? - (2012-08-29)
  [3766] Python timing - when to use a list, and when to use a generator - (2012-06-16)
  [3695] Functions are first class variables in Lua and Python - (2012-04-13)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3474] Python Packages - groupings of modules. An introduction - (2011-10-11)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3459] Catching the fishes first? - (2011-09-27)
  [3280] Passing parameters to Python functions - the options you have - (2011-05-07)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2766] Optional and named parameters to Python functions/methods - (2010-05-15)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [2506] Good example of recursion in Python - analyse an RSS feed - (2009-11-18)
  [2481] Sample code with errors in it on our web site - (2009-10-29)
  [2440] Optional parameters to Python functions - (2009-10-07)
  [2439] Multiple returns from a function in Python - (2009-10-06)
  [2011] Conversion of OSI grid references to Eastings and Northings - (2009-01-28)
  [1879] Dynamic code - Python - (2008-11-11)
  [1871] Optional and named parameters in Python - (2008-11-05)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1790] Sharing variables with functions, but keeping them local too - Python - (2008-09-09)
  [1464] Python Script - easy examples of lots of basics - (2007-12-08)
  [1134] Function / method parameters with * and ** in Python - (2007-04-04)
  [959] It's the 1st, not the 1nd 1rd or 1th. - (2006-12-01)
  [949] Sludge off the mountain, and Python and PHP - (2006-11-27)
  [913] Python - A list of methods - (2006-11-03)
  [912] Recursion in Python - (2006-11-02)
  [900] Python - function v method - (2006-10-20)
  [821] Dynamic functions and names - Python - (2006-08-03)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [745] Python modules. The distribution, The Cheese Shop and the Vaults of Parnassus. - (2006-06-05)
  [668] Python - block insets help with documentation - (2006-04-04)
  [561] Python's Generator functions - (2006-01-11)
  [418] Difference between import and from in Python - (2005-08-18)
  [386] What is a callback? - (2005-07-22)
  [303] Lambdas in Python - (2005-05-06)
  [294] Python generator functions, lambdas, and iterators - (2005-04-28)
  [105] Distance Learning - (2004-10-31)

Z203 - Cat and Dog
  [4666] Spring at Well House Manor - (2016-03-27)
  [4526] Day trip to Lancaster - (2015-10-10)
  [4517] Black dog day - and Bobby still seeks his new home - (2015-09-26)
  [4507] In favour of adoption rather than puppy purchase - dogs! - (2015-06-07)
  [4433] Different views of a Welsh Valley - but headed home - (2015-02-20)
  [4417] Crisp morning, fast run - (2015-02-06)
  [4333] Misty Melksham Morning - (2014-11-30)
  [4316] Too many Staffies, too many lurchers - (2014-11-16)
  [4189] Who are we - Lisa, Graham, Gypsy and Billy - (2013-10-07)
  [4180] Exceptionally, I sign a petition - on a life and death issue for Irish dogs - (2013-09-20)
  [4130] Soft furnishings up a Welsh Mountain - (2013-07-03)
  [4016] Red, yellow, green or pink dog? Yellow Dog Project - (2013-02-23)
  [3943] With us for a year already! - (2012-12-03)
  [3936] Lisa is home today ... - (2012-11-26)
  [3920] Challenge for a photographer - (2012-11-10)
  [3908] Black dogs at Halloween - (2012-10-31)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3737] Postcards from Barmouth - (2012-05-20)
  [3675] Spring 2012 - (2012-03-30)
  [3674] Off to walk the dogs - (2012-03-30)
  [3625] An update on Billy the Greyhound and Gypsy the Lurcher - (2012-02-25)
  [3606] Gypsy and Billy enjoy the snow - (2012-02-10)
  [3561] Happy Christmas ... a Christmas morning walk in Melksham - (2011-12-25)
  [3553] Changes to morning routines - (2011-12-16)
  [3541] I loves Melksham - (2011-12-06)
  [3537] Letting the foster parents know ... too little or too much? - (2011-12-04)
  [3533] No message - just doggy pictures of the new doggy - (2011-11-30)
  [3530] A typical weekend?? - (2011-11-28)
  [3338] Return trip - Dogs Trust, Newbury - (2011-06-27)
  [3324] Rabbit Shelter - (2011-06-12)
  [3096] Contrast in pictures - (2010-12-20)
  [2897] The Land of the Black Labrador - (2010-08-01)
  [2787] Sleeping in on Sunday - (2010-05-30)
  [2689] Can my dog eat potatoes? Doggie Dietary Research, and political sleaze! - (2010-03-23)
  [2555] Bookkeeping - (2009-12-29)
  [2499] ourdog is Greyhound, Staffie and Ginger Cake - (2009-11-09)
  [2496] A better design of mouth - (2009-11-07)
  [2456] Bright Eyes - (2009-10-16)
  [2451] Gypsy says - (2009-10-12)
  [2450] Family Gathering at 404, The Spa - (2009-10-11)
  [2295] The dog is not in trouble - (2009-07-17)
  [2271] Dogs Trust, Dog Show, Newbury - (2009-07-06)
  [2252] Leaping dog, Leaping horse, copyright of old masters - (2009-06-20)
  [2167] Watching the tele - (2009-05-10)
  [2140] Beware - giving copyright away when you upload a picture - Dogs Trust - (2009-04-25)
  [2138] Ski bore ... train bore ... dog bore - (2009-04-24)
  [2132] Learning about Lurchers - (2009-04-18)
  [2029] Also for the eyes of the cat - (2009-02-07)
  [771] From cat breeder to Cobol to Perl - (2006-06-21)
  [744] We can offer a room, but we can't operate on a dog - (2006-06-05)
  [547] Taking the dog for a walk - (2005-12-29)
  [62] Technical Open House - (2004-09-25)


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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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