From: Paul Lutus <nospam@...>
Date: 2006-10-04T01:10:06+09:00
Subject: Re: sparse multi-dimensional arrays

Bill Birkett wrote:

> I did like to convert some programs I wrote in Perl to Ruby. I'm
> working with sparse three dimensional data. Here's an example in
> Perl. The array @a has just two elements. The rest are undefined.
> 
> #!/usr/bin/perl -w
> 
> $a[0][0][0] = 0;
> $a[100][100][100] = 100;
> 
> print "$a[0][0][0]\n";
> print "$a[100][100][100]\n";
> 
> The output:
> 
> 0
> 100
> 
> Is there an easy way to implement this sort of data  structure in
> Ruby? The Array class seems limited to one dimension. I looked at the
> Matrix class, but there doesn't appear to be any way of assigning
> values to the individual matrix elements (which seems pretty strange,
> so maybe I'm overlooking the obvious).

The solution depends on what you are trying to accomplish. If your aim is to
minimize storage space, then use a nested hash, with numeric indices as
keys. If your aim is simply to create an n-dimensional array and you don't
care about storage size, then follow the advice of another poster:

#!/usr/bin/ruby -w

dim_size = 100

arr = Array.new(dim_size) { Array.new(dim_size) { Array.new(dim_size,42) } }

puts arr[39][56][24]

42

-- 
Paul Lutus
http://www.arachnoid.com