How to reshape an array with an arbitrary size in one dimension?

Question

How would you go about reshaping a length-N 1D array into an (N/2) x 2 array? In numpy, for example, you can use -1 in place of N/2 when reshaping:

>>> x = np.array([1,2,3,4])
>>> x.reshape((-1, 2))
array([[1, 2],
       [3, 4]])

So I'm wondering if there's a J equivalent. It feels clunky to explicitly calculate the required number of rows.

Answer 1

This is what infix (\) is for: n ,\y will ravel (,) every n items ("infixes" of length n). If n is negative the infixes are non overlapping.

y =. i.20

_2 ,\y
 0  1
 2  3
 4  5
 6  7
 8  9
10 11
12 13
14 15
16 17
18 19

_3 ,\y
 0  1  2
 3  4  5
 6  7  8
 9 10 11
12 13 14
15 16 17
18 19  0