From: Larry Evans <cppljevans@...>
Date: 2011-01-10T14:10:18+09:00
Subject: Re: Nooby question : multidimensional arrays.

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On 01/08/11 13:33, Larry Evans wrote:
> On 01/07/11 21:57, Larry Evans wrote:
>> On 01/07/11 17:56, Kedar Mhaswade wrote:
>>> Is this an okay job of implementing a multidimensional array?
>>>
>>> https://github.com/kedarmhaswade/datacube/blob/master/data_cube.rb
>>>
>>> Thank you for any feedback.
>>>
>>> -Kedar
>>>
>> There's a book:
>>
>>   http://web.engr.oregonstate.edu/~budd/Books/aplc/
>>
>> which describes an expansion vector which is a scan of
>> the array sizes.  IOW, for:
>>
>>   arr = Array.new(s0,s1,...sn)
>>
>> the expansion vector for this arr is:
>>
>>   arr.ev = [1, s0, s0*s1, s0*s1*s3,..., s0*s1*...*sn]
>>
> The expansion vector for DataCube.new(n,m) would be:
> 
>   [m**0,m**1,m**2,...,m**n]
> 
> IOW, the length of this expansion vector is n+1.
> 
> [snip]
>>
>>     arr[i0,i1,..., ij, ... in]
>>
>> is located:
>>
>>     i0*arr.ev[0]+i1*arr.ev[1]+...+in*arr.ev[n]
>>
>> elements from the 1st element:
>>
>>     arr[0,0,...,0]
>>
>> Thus, given the sizes, you can create a member variable
>> which is the expansion vector, then use that to access the
>> elements by calculating the offset from the initial element
>> using the dot product:
>>
>>     i0*arr.ev[0]+i1*arr.ev[1]+...+in*arr.ev[n]
>>
[snip]

To illustrate, the attached produces output:

44
12
[0,0,0]
0
[1,0,0]
1
[0,1,0]
2
[1,1,0]
3
[1,1,2]
11
[ [ [0, 1]
  , [2, 3]
  ]
, [ [4, 5]
  , [6, 7]
  ]
, [ [8, 9]
  , [44, 11]
  ]
]

The last several lines (starting with [ [ [)
show the output to the to_s and were designed
to show (hopefully) the nested subarray's.

-regards,
Larry

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