[Tutor] Advice on multi-dimensional data storage

[Steven D'Aprano]

On Wed, Mar 16, 2016 at 08:36:59AM +0000, Matt Williams wrote:
> Dear Tutors,
> 
> I am looking for some advice. I have some data that has three dimensions to
> it. I would like to store it such that one could manipulate (query/ update/
> etc.) by dimension - so it would be feasible to ask for all of the data
> that shares a value in d1, or iterate over all of the values via d2.

As a beginner, you should start with the simplest thing that works: 
nested lists. If that's not suitable, perhaps too slow, then start to 
look into more complex solutions, like numpy arrays, or a database.

Here's a two dimensional array using nested lists:

arr = [ [1, 2, 3, 4],
        [5, 6, 7, 8],
        [9, 10, 11, 12] ]

Three rows x 4 columns. Easy, right? Now let's make it three 
dimensional. It looks a bit uglier, but that's because we're trying to 
write a three dimensional array in two dimensional text:


arr3D = [
         # Block 0, 3 rows by 4 columns.
         [ [1, 2, 3, 4],
           [5, 6, 7, 8],
           [9, 10, 11, 12] ],
         # Block 1.
         [ [0, 0, 0, 0],
           [1, 2, 4, 8],
           [2, 4, 6, 8] ],
         # Block 2.
         [ [0, 1, 2, 3],
           [1, 2, 4, 8],
           [2, 6, 10, 14] ]
         ]


Remember that Python counts positions starting from zero, not one, so it 
is row 0, 1, 2 not row 1, 2, 3.

To access an individual item, give each index in square brackets:

py> arr3D[0][2][3]  # block 0, row 2, column 3
12


Return an entire block:

py> arr3D[1]  # block 1 is 3 rows x 4 columns
[[0, 0, 0, 0], [1, 2, 4, 8], [2, 4, 6, 8]]


Return an entire row:

py> arr3D[1][2]  # row 2 of block 1
[2, 4, 6, 8]


There's no easy way to return a column, but we can use a "list 
comprehension" to extract one out of a block:

py> [row[3] for row in arr3D[1]] # extract column 3 of block 1
[0, 8, 8]


Hope this helps!



-- 
Steve