[Numpy-discussion] how to efficiently build an array of x, y, z points
David Goldsmith
d.l.goldsmith at gmail.com
Tue Mar 2 22:47:46 EST 2010
On Tue, Mar 2, 2010 at 6:59 PM, Brennan Williams <
brennan.williams at visualreservoir.com> wrote:
> David Goldsmith wrote:
> >
> > On Tue, Mar 2, 2010 at 6:29 PM, Brennan Williams
> > <brennan.williams at visualreservoir.com
> > <mailto:brennan.williams at 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?
>
My guess is that either way will be better than iteratively "appending" to
an existing 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.
>
Great! Be sure to report back. :-)
Dg
>
> Brennan
> > DG
> >
> >
> >
> > Thanks
> >
> > Brennan
> >
> >
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