On Mon, Nov 9, 2009 at 7:59 PM, <josef.p...@gmail.com> wrote: > 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.
on a related note: I looked at the doc editor discussion for numpy.where and it seems broadcasting has been fixed also for where (in 1.3.0). I don't know whether the c implementation is related to choose. I only tried Paulis example. Josef > > 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
