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

by Marshall (Prior)
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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