On Mon, Nov 9, 2009 at 7:54 PM, David Goldsmith <d.l.goldsm...@gmail.com> wrote: > May I infer from the sudden silence that I finally have it?
I think so, I assume that the result of broadcasting is unique, I haven't seen an example yet where broadcasting would depend on the sequence in which it is done. Josef > > DG > > On Sun, Nov 8, 2009 at 8:50 PM, David Goldsmith <d.l.goldsm...@gmail.com> > wrote: >> >> OK, let me see if I'm interpreting this example correctly: >> >> >>> c1=np.arange(2).reshape(2,1,1); c1 >> array([[[0]], >> >> [[1]]]) >> >>> c2=2+np.arange(2).reshape(1,1,2); c2 >> array([[[2, 3]]]) >> >>> a=np.eye(2,dtype=int) >> >>> np.choose(a, [c1, c2]) >> array([[[2, 0], >> [0, 3]], >> >> [[2, 1], >> [1, 3]]]) >> >> First, everything is being broadcast to (2,2,2); a is broadcast to >> [[[1,0], [0,1]], [[1,0], [0,1]]], c1 is broadcast to [[[0,0], [0,0]], >> [[1,1], [1,1]]] and c2 is broadcast to [[[2,3], [2,3]], [[2,3], [2,3]]]. >> Now result is created by "stepping through" broadcast a and using, >> respectively, the positionally corresponding element from broadcast c1 >> (resp. c2) if the value in a at the position is 0 (resp. 1). At least, this >> gives the result above (but I have not examined other possible broadcasts of >> the arguments to see if they would also give the result - I conjectured what >> appeared to me to be the most "natural" broadcasts and checked to see if it >> worked and it does; is there something I should know - e.g., uniqueness of >> the result, or a rule governing how choose broadcasts - to *know* that the >> broadcasts above are indeed the broadcasts choose is using?) >> >> Thanks again, >> >> DG >> On Sun, Nov 8, 2009 at 8:19 PM, Anne Archibald <peridot.face...@gmail.com> >> wrote: >>> >>> 2009/11/8 David Goldsmith <d.l.goldsm...@gmail.com>: >>> > On Sun, Nov 8, 2009 at 7:40 PM, Anne Archibald >>> > <peridot.face...@gmail.com> >>> > wrote: >>> >> >>> >> As Josef said, this is not correct. I think the key point of confusion >>> >> is >>> >> this: >>> >> >>> >> Do not pass choose two arrays. >>> >> >>> >> Pass it one array and a *list* of arrays. The fact that choices can be >>> >> an array is a quirk we can't change, but you should think of the >>> >> second argument as a list of arrays, >>> > >>> > Fine, but as you say, one *can* pass choose an array as the second >>> > argument >>> > and it doesn't raise an exception, so if someone is stupid/careless >>> > enough >>> > to pass an array for `choices`, how is choose interpreting it as a >>> > list? Is >>> > the first dimension "list converted" (so that, e.g., my (2,1,2) example >>> > is >>> > interpreted as a two element list, each of whose elements is a (1,2) >>> > array)? >>> >>> It seems to me that this is the only reasonable interpretation, yes. >>> After all, arrays behave like sequences along the first axis, whose >>> elements are arrays of one less dimension. Thus if you pass an array, >>> any broadcasting happens ignoring the first axis, which is a rather >>> abnormal pattern for numpy broadcasting, but necessary here. >>> >>> As a bonus, I think this is what is implemented in current versions of >>> numpy. (In 1.2.1 it raises an exception if broadcasting is necessary.) >>> >>> Anne >>> _______________________________________________ >>> NumPy-Discussion mailing list >>> NumPy-Discussion@scipy.org >>> http://mail.scipy.org/mailman/listinfo/numpy-discussion >> > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
