Sort Multiple Lists Together by a Key Row

Question

Given:

d:(5 3 1 2 4;`s`t`y`a`i)

Reorder both rows together, so that the first row is sorted ascending.

Expected result:

1 2 3 4 5
`y`a`t`i`s

Solution

d@\:iasc d[0]

Explanation

iasc d[0]

returns the indexes that would sort the first row ascending — not the sorted values themselves, but the order needed to get there:

q)iasc d[0]
2 3 1 4 0

The each-left iterator:

\:

then pairs that fixed index list against every row of d in turn, indexing each row independently with the same index order:

d[0]@2 3 1 4 0   / 1 2 3 4 5
d[1]@2 3 1 4 0   / `y`a`t`i`s

Because both rows are indexed with the same index list, they stay aligned with each other — the symbol that was paired with 5 (the largest number) is still paired with it after sorting, just relocated to the end. This pattern is how parallel rows or columns are kept in sync in q without sorting them separately.