Mar 21, 2011

Given a value, how do I get the null of that value’s type?

We take advantage of a property of q’s indexing: If you request an item from a list (or a dictionary, for that matter) using an index that is out of range, q returns the null item for the list’s type.

q)x: 1 2 3
q)x 3
0N
q)

Thus, we can take the item whose corresponding null we want, make a list from it, and then access that list with an out-of-range index:

q)NullOf: {[item] enlist[item] 1}
q)NullOf 1
0N
q)(::) ~ NullOf {x*x}
1b
q)null NullOf (.z.D; 1e; ” “; type “foo”)
1111b
q)

Remember, there is no null list. If you pass a list to NullOf, you get the empty list of the corresponding type:

q)NullOf 1 2 3
`int$()
q)

See also: .Q.ty

See Also