Global Vars

Melly has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monkeys,

I've got a big chunk of code for some data-transformation. Basically, it reads in some mapping information, some config options, and then processes a hefty data-file.

This data-file consists of n number of lines per subject, and the process handles each subject, reading, translating and outputting data, and then moves on to the next subject.

Now, all the subject-specific stuff uses local variables, and passes them to sub-routines in the approved fashion.

However, the mapping and config options are all stored in global vars.

How much of this is a no-no IYHO? (ONSHO?)

map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2
-$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
[download]

Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk

Comment on Global Vars Download Code

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Re: Global Vars by roboticus (Chancellor) on Sep 04, 2009 at 13:36 UTC
Melly: For quick & dirty stuff, I don't mind globals a bit. They get to be a problem when the program starts to get too large and complicated. If you ever find yourself using `grep` to track down usage of a global variable, or when you have a quick reference sheet for your global variables, or when you track down a bug due to reusing a global variable name that you forgot, then you know that your program is past due for some reorganization/refactoring/etc. I always wonder why quick & dirty programs last longer than the supposedly long-term ones? ...roboticus	[reply] [d/l]
Re^2: Global Vars by tbone1 (Monsignor) on Sep 04, 2009 at 13:57 UTC
What he said. I have sometimes created a global hash (called something like %my_global_values or something like that) so that 1) it's obvious what is a global variable, and 2) I'm far less likely to accidentally reuse a variable. And the quick and dirty programs probably last longer because expectations are lower, they're for (relatively) simple things, and developers are less likely to overthink them. -- tbone1, YAPS (Yet Another Perl Schlub) And remember, if he succeeds, so what. - Chick McGee	[reply]
Re^3: Global Vars by roboticus (Chancellor) on Sep 04, 2009 at 15:01 UTC
tbone1: Nice technique. It seems to me that it's a handy step on the way to splitting things off into modules and/or object-oriented bits. Since all the references go through your global hash anyway, when you split off a function/module/object, the code in your subroutines wouldn't have to change much: `BEFORE` `my %my_global_values = (foo=>0, etc=>0); my $rGlobals = \%my_global_values; sub zigafoo { $rGlobals-> = 7 }` [download] `AFTER` `package barbaz; my %my_package_values = (foo=>0); my $rPackage = \%my_package_values; sub foobar { $rPackage->{foo} = 7; }` [download] ...roboticus	[reply] [d/l] [select]
Re: Global Vars by JavaFan (Canon) on Sep 04, 2009 at 14:52 UTC
It depends what you use the variable for. If you use it globally, by all means, use a global variable. That's, IMO, cleaner than passing the same variable to (almost) every function. It's even worse if you have 12 such variables.	[reply]
Re^2: Global Vars by Melly (Chaplain) on Sep 04, 2009 at 16:13 UTC
Thanks all - pretty much confirms what I thought. Phew. `map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2$d$e+$a,$e2 -$d2+$b);$c=$d2+$e2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20` [download] Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk	[reply] [d/l]


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