In the following output (see below), why would x[1,None] work, but x[1,None,2] or even x[1,2,None] not work?
Incidentally, I would be very interested in a solution that allows me to index numpy arrays using a list/iterable that might contain None values. Is there a straightforward way to do so, or should I just use list comprehensions (i.e. [x[a] for a in indices if a])?
Thanks! Orest
In [122]: x = arange(5)
In [123]: x[1] Out[123]: 1
In [124]: x[None] Out[124]: array([[0, 1, 2, 3, 4]])
In [125]: x[1,None] Out[125]: array([1])
In [126]: x[1,None,2] --------------------------------------------------------------------------- <type 'exceptions.IndexError'> Traceback (most recent call last)
c:\documents\research\programs\python\epl\src<ipython console> in <module>()
<type 'exceptions.IndexError'>: invalid inde
On 9/9/07, Orest Kozyar orest.kozyar@gmail.com wrote:
In the following output (see below), why would x[1,None] work, but x[1,None,2] or even x[1,2,None] not work?
None is the same thing as newaxis (newaxis is just an alias for None). Armed with that tidbit, a little perusing of the docs should quickly explain the behaviour you are seeing.
Incidentally, I would be very interested in a solution that allows me to
index numpy arrays using a list/iterable that might contain None values. Is there a straightforward way to do so, or should I just use list comprehensions (i.e. [x[a] for a in indices if a])?
You could try something using compress x[compress(indices, indices)] for example. Whether that's superior to a iterator solution would probably depend on the problem. I'd probably use fromiter instead of a list comprehension if I went that route though. There are probably other ways too. It very much depends on the details and performance requirements of what you are trying to do.
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Orest Kozyar -
Timothy Hochberg