[PYTHON MATRIX-SIG] Creating grid object

[David Ascher]

> Hello, I want to use Numpy (is this name "official"?) to build an
> analysis package for a numerical model. At the moment I implement some
> simple functions, which are working on array-arguments. Now I get the
> idea, that it would nice to have an array-object of a variable say
> salinity, with an attribut about the edges of the grid the variable is
> defined on, so that I can work on arbitray blocks and can get the
> information where this block is later.

I didn't see any response to this message, so I'll give it a shot.

I'm not sure I entirely understand your requirements, but here's what I
think I read in your message. 

     You want to define a new object type (call it a class) which 
     has some of the attributes of a python multiarray, and some
     attributes which are specific to your application (salinity, etc.).

This I would do by subclassing the UserArray class which is part of the
Lib directory of NumPy.  It is basically a class wrapper around the
builtin python type.

> I want to define the hole grid as a global variable and let the edges
> in the new object point at a subregion of the grid. I'm new to python
> and object-oriented programming, but I think that I must define my own
> class. My problem now is, that I want to do normal math directly with
> this object, like
> 
>     sal_anomaly_on_grid = sal_on_grid - mean_sal_in_depth_on_this_grid
> 
> 	 ^new object        ^new object     ^normal 1d array
> 
> At the moment I know only to do something like
> 
>     sal_anomaly_on_grid = sal_on_grid.data - mean_sal_in_depth_on_this_grid
> 
> 	^normal array       ^new object        ^normal array
> 
> My grid is defined in two 2d-arrays x_loc,y_loc and one 3d-array  z with
> depth of x,y in n layers. My first idea is to have an attribut in the new
> object, wich is a list of 16 points
> 
> 	x_loc(index_of_x_starting_location)
> 	x_loc(index_of_y_starting_location)
> 	x_loc(index_of_x_stop_location)
> 	x_loc(index_of_y_stop_location)
> 	4 y_loc(...)
> 	8 z
> 
> But it would be better, to have references to each gridpoint, to allow
> more flexibel grids, but this is perhaps memory-consuming. 

Ah, I think I understand better now.  You want to have a big array which
stores all your data, and then have other objects which are in some ways
like 'enhanced slices'.  In other words, they point to a subset of the
big array, but they are not just slices because they have other
properties.  In other words, you want a UserSlice object, I think.  That
doesn't exist to my knowledge, but it certainly seems a worthy project.
Am I paraphrasing your needs correctly?  I suspect that the key to doing
this is understanding UserArray.py and slice() well.  Remember that the
slice() operator returns an 'array by reference':

	>>> a = arange(10)
	>>> a
	0 1 2 3 4 5 6 7 8 9
	>>> b = a[3:5]
	>>> b
	3 4 
	>>> a[3] = 0
	>>> a 
	0 1 2 0 4 5 6 7 8 9
	>>> b
	0 4

[Jim: is UserArray up to date?]

> Is it
> possible to build something like this?  Is the definition of such
> classes covered in one of the new python-books? Or must I program
> something in C (I hope not).

I suspect that Mark and Aaron's books cover class creation, maybe
'class-around-type' creation, but probably not the specific numeric
array object.  I don't think you need to do any C for this.

> Last but not least, is this group the right place to ask? I think I
> will have more questions in the future.

Yes.

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