Question

If I have a square matrix as a nested list in python I can split it up into several equal sized boxes and create a new list where each element is a list of the numbers in one of those boxes. E.g.

a = [[1, 2, 3, 4],
     [5, 6, 7, 8],
     [9, 10, 11, 12],
     [13, 14, 15 ,16]]

b = [[a[0][0], a[0][1], a[1][0], a[1][1]],
     [a[0][2], a[0][3], a[1][2], a[1][3]],
     [a[2][0], a[2][1], a[3][0], a[3][1]],
     [a[2][2], a[2][3], a[3][2], a[3][3]]]

Is there an easier way to do this? Is there a way to set this up as a function which I can apply to matrices of different sizes and also specify the size of the boxes?

Answer 1

The following is equivalent to what you have and a bit more concise:

b = [a[0][:2] + a[1][:2],
     a[0][2:] + a[1][2:],
     a[2][:2] + a[3][:2],
     a[2][2:] + a[3][2:]]

Or an equivalent list comprehension:

b = [a[i][s] + a[i+1][s] for i in (0,2) for s in (slice(None,2), slice(2,None))]