[Numpy-discussion] how to efficiently build an array of x, y, z points
Brennan Williams
brennan.williams@visualreservoir....
Tue Mar 2 20:59:02 CST 2010
David Goldsmith wrote:
>
>
> On Tue, Mar 2, 2010 at 6:29 PM, Brennan Williams
> <brennan.williams@visualreservoir.com
> <mailto:brennan.williams@visualreservoir.com>> wrote:
>
> I'm reading a file which contains a grid definition. Each cell in the
> grid, apart from having an i,j,k index also has 8 x,y,z coordinates.
> I'm reading each set of coordinates into a numpy array. I then want to
> add/append those coordinates to what will be my large "points" array.
> Due to the orientation/order of the 8 corners of each hexahedral
> cell I
> may have to reorder them before adding them to my large points array
> (not sure about that yet).
>
> Should I create a numpy array with nothing in it and then .append
> to it?
> But this is probably expensive isn't it as it creates a new copy
> of the
> array each time?
>
> Or should I create a zero or empty array of sufficient size and
> then put
> each set of 8 coordinates into the correct position in that big array?
>
> I don't know exactly how big the array will be (some cells are
> inactive
> and therefore don't have a geometry defined) but I do know what its
> maximum size is (ni*nj*nk,3).
>
>
> Someone will correct me if I'm wrong, but this problem - the "best"
> way to build a large array whose size is not known beforehand - came
> up in one of the tutorials at SciPyCon '09 and IIRC the answer was,
> perhaps surprisingly, build the thing as a Python list (which is
> optimized for this kind of indeterminate sequence building) and
> convert to a numpy array when you're done. Isn't that what was
> recommended, folks?
>
Build a list of floating point values, then convert to an array and
shape accordingly? Or build a list of small arrays and then somehow
convert that into a big numpy array?
I've got 24 floating point values which I've got in an array of shape
(8,3) but I could easily have those in a list rather than an array and
then just keep appending each small list of values to a big list and
then do the final conversion to the array - I'll try that and see how it
goes.
Brennan
> DG
>
>
>
> Thanks
>
> Brennan
>
>
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