[Numpy-discussion] question on use of newaxis

Friedrich Romstedt friedrichromstedt at gmail.com
Wed Feb 9 05:40:21 EST 2011

2011/2/9 Mark Bakker <markbak at gmail.com>:
> a = array([1,2,3])
> I want to take the last two terms and make it a column vector:
> a[[1,2],newaxis]
> a[[1,2]][:,newaxis]

The former is advanced indexing, while the latter is basic slicing
(after advanced indexing).  See
There, in the advanced indexing case, "All sequences and scalars in
the selection tuple are converted to intp indexing arrays", and:

>>> numpy.intp
<type 'numpy.int32'>

.  Thus it tries to interpret the None:

>>> numpy.newaxis
>>> print repr(numpy.newaxis)

as an ``numpy.intp``, hence the error.  Since advanced indexing
deletes axes of an ndarray and inserts new ones (see
last point), a ``numpy.newaxis`` might be ambiguous.  In your case, it
would be not, though.


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