Wrap slice around edges of a 2D array in numpy
numpy, python
Solution
This will work with numpy >= 1.7.
a = np.arange(25).reshape(5,5)
array([[ 0, 1, 2, 3, 4],
[ 5, 6, 7, 8, 9],
[10, 11, 12, 13, 14],
[15, 16, 17, 18, 19],
[20, 21, 22, 23, 24]])
The pad routine has a 'wrap' method...
b = np.pad(a, 1, mode='wrap')
array([[24, 20, 21, 22, 23, 24, 20],
[ 4, 0, 1, 2, 3, 4, 0],
[ 9, 5, 6, 7, 8, 9, 5],
[14, 10, 11, 12, 13, 14, 10],
[19, 15, 16, 17, 18, 19, 15],
[24, 20, 21, 22, 23, 24, 20],
[ 4, 0, 1, 2, 3, 4, 0]])
Depending on the situation you may have to add 1 to each term of any slice in order to account for the padding around `b`.
Problem
Suppose I am working with numpy in Python and I have a two-dimensional array of arbitrary size. For convenience, let's say I have a 5 x 5 array. The specific numbers are not particularly important to my question; they're just an example. ``` a = numpy.arrange(25).reshape(5,5) ``` This yields: ``` [[0, 1, 2, 3, 4 ], [5, 6, 7, 8, 9 ], [10,11,12,13,14], [15,16,17,18,19], [20,21,22,23,24]] ``` Now, let's say I wanted to take a 2D slice of this array. In normal conditions, this would be easy. To get the cells immediately adjacent to 2,2 I would simply use `a[1:4,1,4]` which would yield the expected ``` [[6, 7, 8 ], [11, 12, 13], [16, 17, 18]] ``` But what if I want to take a slice that wraps around the edges of the array? For example `a[-1:2,-1:2]` would yield: ``` [24, 20, 21], [4, 0, 1 ], [9, 5, 6 ] ``` This would be useful in several situations where the edges don't matter, for example game graphics that wrap around a screen. I realize this can be done with a lot of if statements and bounds-checking, but I was wondering if there was a cleaner, more idiomatic way to accomplish this. Looking around, I have found several answers such as this: https://stackoverflow.com/questions/17739543/wrapping-around-slices-in-python-numpy that work for 1-dimensional arrays, but I have yet to figure out how to apply this logic to a 2D slice. So essentially, the question is: how do I take a 2D slice of a 2D array in numpy that wraps around the edges of the array? Thank you in advance to anyone who can help.