[Numpy-discussion] Slicing a numpy array and getting the "complement"

[Anne Archibald]

2008/5/19 Orest Kozyar <orest.kozyar@gmail.com>:
> Given a slice, such as s_[..., :-2:], is it possible to take the
> complement of this slice?  Specifically, s_[..., ::-2].  I have a
> series of 2D arrays that I need to split into two subarrays via
> slicing where the members of the second array are all the members
> leftover from the slice.  The problem is that the slice itself will
> vary, and could be anything such as s_[..., 1:4:] or s_[..., 1:-4:],
> etc, so I'm wondering if there's a straightforward idiom or routine in
> Numpy that would facilitate taking the complement of a slice?  I've
> looked around the docs, and have not had much luck.

If you are using boolean indexing, of course complements are easy
(just use ~). But if you want slice indexing, so that you get views,
sometimes the complement cannot be expressed as a slice: for example:

A = np.arange(10)
A[2:4]

The complement of A[2:4] is np.concatenate((A[:2],A[4:])). Things
become even more complicated if you start skipping elements.

If you don't mind fancy indexing, you can convert your index arrays
into boolean form:
complement = A==A
complement[idx] = False

Anne