[Brisbane-pm] Passing references into Arrays

[Martin Jacobs]

Hi folks,

Having read through the relevant parts of 'Programming Perl', 'Perl  
Best Practices' and PerlRefTut, I am still getting to grips with  
referencing and dereferencing, and I'm trying to formulate some  
conventions to make my code readable (or at least internally  
consistent). With this in mind, I've been trying to establish a 'one  
size fits all' way to pass variables into subs. I know that this  
might be inefficient for subs with only a couple of variables, but  
some of my subs need a dozen or so variables. Furthermore, I need to  
pass references. I'm going to be dealing with some arrays that are 7  
million lines long, so I don't want to copy the arrays if I can help  
it. The data structures of the arrays are simple, its just that they  
are big.

In following 'Perl Best Practices', I have set up the attached two  
versions of a 'conventional' program, which I intend to use as a kind  
of template for my 'real' program (its called PERRMOSS). They  
actually do what they are supposed to do, though I don't have the  
experience to fully explain why. I would be grateful for some sanity  
checking here.

If you've been reading my earlier posts, you will note that I'm not  
trying to use prototypes in these programs.

A couple of questions;

Perl Best Practices says 'Use a hash of named arguments for any  
subroutine that has more than three parameters'. OK, that's PERRMOSS.  
I have varied the advice, though, in that I have set up a hash of  
references to the variables (because some of the arrays will be very  
big). This does seem rather convoluted because you've got to set up a  
hash with all the keys, then pass the hash to the sub, then look in  
all the keys in the sub to find what you're looking for. Does this  
approach make more defensive programming, or less? If the keys are to  
make sense, then they should have long names, like the names of the  
values, but then there's the danger of mixing up the key with the value.

Version 1 works well, but the syntax is cluttered because each  
operation has to derefence each variable at every step.

Version 2 attempts to get round this by dereferencing everything at  
the start of the sub. Does this mean it copies the variables in the  
sub?  Oddly, and I don't know why, if you hash out line 31, the sub  
applies the operation to the referent outside the sub, @k.  I thought  
that the referent would not change until you get to the end of the  
sub, which is what happens for $i, @j or %l. I take this as  
signifying that the code is not robust, so it is possibly not a good  
strategy to pursue.

Thanks in advance for any help.

(Yes, I know the testprint line numbers in Version 2 are not accurate)

Regards,
Martin
Visit my website...
http://web.mac.com/martin_jacobs1

 
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