Mar 16, 2011

How can I slice a vector into a series of vectors of length n?

By passing the # (take) operator a pair as its first (i.e., left) argument:

q)2 5 # til 10
0 1 2 3 4
5 6 7 8 9
q)

When # is used in this way, it is referred to as “reshape.” We like to call this, “taking a box.” Just like when you apply # with an integer left argument, taking a box stops when you have taken as many items as requested, leaving the rest behind:

q)3 3 # til 10
0 1 2
3 4 5
6 7 8
q)

Similarly, when you over-take a box, you start over from the beginning of the source:

q)4 3 # til 10
0 1 2
3 4 5
6 7 8
9 0 1
q)

If you want to take all the items from the source, pass 0N as the first element of the box:

q)0N 2 # til 10
0 1
2 3
4 5
6 7
8 9
q)

If you take a whole box, but the elements from the source don’t fit, you end up with a short list in the last row:

q)0N 3 # til 10
0 1 2
3 4 5
6 7 8
,9
q)

See Also