Find Sublists Containing a Value

Question

Given the following list of lists:

l:(1 2 3 4;7 2 4 1;1 3 5 1 8;0 8;8 2 4)

Write a q expression that returns all sublists which contain the number 2.

Expected result:

1 2 3 4
7 2 4 1
8 2 4

Solution

l where 2 in/: l

Explanation

The each-right iterator:

/:

pairs a single left argument against every item on the right, one at a time. Here the fixed left argument 2 is tested with in against every sublist of l independently:

q)2 in/: l
11001b

producing one boolean result per sublist — 1b wherever that sublist contains 2. where then keeps only the sublists for which this is true.

each-right is the mirror image of each-left: instead of a fixed right argument paired against every item on the left, it's a fixed left argument paired against every item on the right.