On Di, 2015-04-07 at 00:49 +0100, Nicholas Devenish wrote: > With the indexing example from the documentation: > > y = np.arange(35).reshape(5,7) > > Why does selecting an item from explicitly every row work as I’d expect: > >>> y[np.array([0,1,2,3,4]),np.array([0,0,0,0,0])] > array([ 0, 7, 14, 21, 28]) > > But doing so from a full slice (which, I would naively expect to mean “Every > Row”) has some…other… behaviour: > > >>> y[:,np.array([0,0,0,0,0])] > array([[ 0, 0, 0, 0, 0], > [ 7, 7, 7, 7, 7], > [14, 14, 14, 14, 14], > [21, 21, 21, 21, 21], > [28, 28, 28, 28, 28]]) > > What is going on in this example, and how do I get what I expect? By > explicitly passing in an extra array with value===index? What is the > rationale for this difference in behaviour? >
The rationale is historic. Indexing with arrays (advanced indexing) works different from slicing. So two arrays will be iterated together, while slicing is not (we sometimes call it outer/orthogonal indexing for that matter, there is just a big discussion about this). These are different beasts, you can basically get the slicing like behaviour by adding appropriate axes to your indexing arrays: y[np.array([[0],[1],[2],[3],[4]]),np.array([0,0,0,0,0])] The other way around is not possible. Note that if it was the case: y[:, :] would give the diagonal (if possible) and not the full array as you would probably also expect. One warning: If you index with more then one array (scalars are also arrays in this sense -- so `[0, :, array]` is an example) in combination with slices, the result can be transposed in a confusing way (it is not that difficult, but usually unexpected). - Sebastian > Thanks, > > Nick > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion
signature.asc
Description: This is a digitally signed message part
_______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
