2011/8/11 Benjamin Root <ben.r...@ou.edu> > > > On Thu, Aug 11, 2011 at 8:37 AM, Olivier Delalleau <sh...@keba.be> wrote: > >> Maybe confusing, but working as expected. >> >> >> When you write: >> matched_to[np.array([0, 1, 2])] = 3 >> it calls __setitem__ on matched_to, with arguments (np.array([0, 1, 2]), >> 3). So numpy understand you want to write 3 at these indices. >> >> >> When you write: >> matched_to[:3][match] = 3 >> it first calls __getitem__ with the slice as argument, which returns a >> view of your array, then it calls __setitem__ on this view, and it fills >> your matched_to array at the same time. >> >> >> But when you write: >> matched_to[np.array([0, 1, 2])][match] = 3 >> it first calls __getitem__ with the array as argument, which retunrs a >> *copy* of your array, so that calling __setitem__ on this copy has no effect >> on your original array. >> >> -=- Olivier >> >> > Right, but I guess my question is does it *have* to be that way? I guess > it makes some sense with respect to indexing with a numpy array like I did > with the last example, because an element could be referred to multiple > times (which explains the common surprise with '+='), but with boolean > indexing, we are guaranteed that each element of the view will appear at > most once. Therefore, shouldn't boolean indexing always return a view, not > a copy? Is the general case of arbitrary array selection inherently > impossible to encode in a view versus a slice with a regular spacing? >
Yes, due to the fact the array interface only supports regular spacing (otherwise it is more difficult to get efficient access to arbitrary array positions). -=- Olivier
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