[Numpy-discussion] lost with slicing

[Partridge, Matthew BGI SYD]

Sorry group.  I found Travis Oliphant's earlier 12 March post (that
didn't show up in the same thread), and found the answer to my question.

matt 

> > > I apologise if I'm asking an obvious question or one that
> > has already
> > > been addressed.
> > >
> > > I've tried to understand the documentation in the numpy manual on 
> > > slicing, but I'm a bit lost.  I'm trying to do indexing 
> using both 
> > > slices and index lists.  I have a problem when I do 
> something like:
> > >
> > > x[0, :, [0,1,2]]
> > >
> > > Here are a couple of examples:
> > >
> > >>>> a = numpy.arange(6).reshape(2,3)
> > >>>> print a
> > > [[0 1 2]
> > >  [3 4 5]]
> > >>>> print a[:, [0,1,2]]   # example 1 - this works as I expected
> > > [[0 1 2]
> > >  [3 4 5]]
> > >>>> b = numpy.arange(6).reshape(1,2,3) print b
> > > [[[0 1 2]
> > >  [3 4 5]]]
> > >>>> print b[0, :, [0,1,2]]  # example 2 - this seems to be the
> > >                              transpose of what I was 
> expecting [[0 
> > > 3]
> > >  [1 4]
> > >  [2 5]]
> > >>>> print b[0, [[0],[1]], [[0,1,2]]] # example 3 - this is what I
> > >                                       expected [[0 1 2]
> > >  [3 4 5]]
> > >
> > > Am I doing something wrong?  Why do we get different behaviour in 
> > > example 2 compared with example 1 or example 3?
> > >
> > > (I'm using numpy 1.0.3.1 on python 2.4.1 for windows, but 
> I've tried 
> > > some more recent versions of numpy as well.)
> > >
> > > mattp
> > >
> > 
> > that's how it works, whether we like it or not.
> > 
> >  see thread with title "is it a bug?" starting  march 11
> > 
> > Josef
> 
> Thanks Josef,
> 
> I've looked over "is it a bug" thread, and realise that it is 
> very relevant!
> But I'm still lost.  Robert Kern wrote:
> 
>   "It's certainly weird, but it's working as designed. Fancy 
> indexing via
>   arrays is a separate subsystem from indexing via slices. Basically,
>   fancy indexing decides the outermost shape of the result (e.g. the
>   leftmost items in the shape tuple). If there are any sliced 
> axes, they
>   are *appended* to the end of that shape tuple."
> 
> I see that's the case in example 2, but not in example 1 
> (above).  Josef, I also see your example doesn't fit this explanation:
> 
>   >>> x = np.arange(30).reshape(3,5,2)
>   >>> idx = np.array([0,1]); e = x[:,[0,1],0]; e.shape
>   (3, 2)
>   >>> idx = np.array([0,1]); e = x[:,:2,0]; e.shape
>   (3, 2)
> 
> Travis Oliphant wrote:
> 
>   Referencing my previous post on this topic.   In this case, it is 
>   unambiguous to replace dimensions 1 and 2 with the result of 
>   broadcasting idx and idx together.   Thus the (5,6) dimensions is 
>   replaced by the (2,) result of indexing leaving the outer dimensions
>   in-tact,  thus (4,2,7) is the result.
> 
> I'm unclear on when something is regarded as "unambiguous"; I 
> don't really get how the rules work.
> 
> I'm trying to build something where I can do (for "a" having 
> a shape (n1,n2,n3,...)):
> 
> a[i1, i2, i3, ...]
> 
> where i1, i2, i3 can be
> * a single index:   eg a[3]
> * a slice:          eg a[:3]
> * a list of keys:   eg a[[1,2,3]]
> and the interpretation of this should yield:
> * no corresponding dimension if a single index is used
> * a dimension of length of the slice if a slice is used
> * a dimension of length of the list if a list is used
> 
> I currently apply the following logic:
> * look through the index coordinates that are being applied
> * if there are multiple list-of-key indices, then reshape 
> them so that they will broadcast to agree:
>    a[[1,2,3], [4,5]] --> a[[[1],[2],[3]], [[4,5]]]
> * note if there are any slices.  If so, I assume (as per 
> Robert Kern's remark) that the dimensions corresponding to 
> the slices are going to be appended to the end.  So I make 
> sure that I transpose my result at the end to correct for this.
> 
> When I do all this, I get example 2 behaving like example 3, 
> but example 1 then doesn't work.  I'm not trying to get the 
> discussion list to do my work for me, but I'm pretty confused 
> as to when dimensions get swapped and when they don't; when 
> something is "ambiguous" and when it is "unambiguous".
> 
> Any help appreciated,
> thanks,
> matt

 
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