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Re^2: why the array index has to start at 0??

by Marshall (Abbot)
on Jun 23, 2009 at 20:47 UTC ( #774170=note: print w/replies, xml ) Need Help??


in reply to Re: why the array index has to start at 0??
in thread why the array index has to start at 0??

A Perl n-D structure is not an "array", certainly not for n>1. To me an array has a fixed, regular memory layout, like a checker or chess board. If I am on row 4, square 6 and I want to know what row 3, square 5 contains, I just go: left 1 and up 1 from where I am at. That's it!

A Perl LoL (List of List), a Perl 2-D structure, doesn't work that way. I've written FORTRAN code with 2-D arrays and some ASM code, but never any C code yet and certainly not any Perl code. It is simply not the way that it is done.

In Perl, every dimension until the last one is a "reference". It works the same as 'C'. If you take a 'C' class, somewhere along the path to the first year, you will learn that this: int x [8][8]; is total BS! There is a HUGE flaw with this because you cannot pass "x" to a subroutine! How big is it? What do I do? The answer to this is similar to how Perl does it. The first dimension is a list of pointers to the 2nd dimension. In the case of a 2-D array, you have to allocate memory for the list of pointers to lists and also for the "rows" themselves and it's a pretty huge hassle!

Anyway what you wind up with is a "list" of pointers to "lists". Now I can give you "x" and tell you to add say 5 to every element in this structure. I don't have to tell you how many rows there are, I don't have to tell you how many columns there are (and they may even vary between rows).

Perl automates a lot of this "grunt work". A Perl 2-D structure is not an array. It is a list of lists.

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[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.
[Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
[hippo]: That's equal parts clever and horrendous.
[Eily]: the overload version wouldn't return true with "$x" && !length $x though, I guess
[hippo]: The more I look at this code, the more $x is a plain old scalar and the more this condition will never be true. I'm calling it a bug at this point.
[hippo]: Thanks for your input which has soothed my sanity (a little)

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