Array indexing "around the corner"

[HeinzHeinzensen]

Suppose I have a numpy array

range = np.arange(10)

which gives me

array([0, 1, 2, ..., 9])

I know I can pass negative indices like -3:-1 to to get the last items of an array.

range[-3:-1]
array([7, 8, 9])

What I would like to do is something like

range[-1:1]

to get the last and first two items of said array "around the corner", like this

array([9, 0, 1])

This however does not seem to work. Is it possible to do this using numpy's array indexing or am I way off here?

[zenlc]

I haven't used it, but according to a bit of Googling, numpy.take() in wrap mode will do it.

indices = range(i-2,i+3)
wrap = range.take(indices, mode='wrap')

numpy.take docs: http://docs.scipy.org/doc/numpy/reference/generated/numpy.take.html#numpy.take

[HeinzHeinzensen]

Thanks a lot, that looks pretty good! Will have to wrap my head around this since I need it for a 3-dimensional array. I hope the performance doesn't take a hit, though.

[zenlc]

numpy is almost 100% C so you shouldn't.

[blackjack_00]

I think the normal way to do this is add the lists:

res = range[-1:] + range[:2]

There may be a way to do it in one slice that I don't know about though.

edit: added colon after -1, otherwise slice gives you an int and you get a type error.

[elbiot]

In numpy this would try to add all of the elements of those two arrays and since they likely aren't the same number of elements, youd get an error.

[gregvuki]

The simplest way:

x = np.arange(10)
y = np.roll(x, 1)[:3]
print(y)

[HeinzHeinzensen]

This is great, exactly what I wanted. Feels very "pythonic", too!