Identity, Null¶
:: Identity¶
Return an argument unchanged
Syntax: (::) x, ::[x]
Returns x.
q)(::)1
1
This can be used in statements applying multiple functions to the same data, if one of the operations desired is "do nothing".
q)(::;avg)@\:1 2 3
1 2 3
2f
Similarly, the identity can also be achieved via indexing.
q)1 2 3 ::
1 2 3
and used in variants thereof for e.g. amends
q)@[til 10;(::;2 3);2+]
2 3 6 7 6 7 8 9 10 11
:: Null¶
Q does not have a dedicated null type. Instead :: is used to denote a generic null value. For example, functions that return no value, return ::.
q)enlist {1;}[]
::
We use enlist to force display of a null result: a pure :: is not displayed.
When a unary function is called with no arguments, :: is passed in.
q)enlist {x}[]
::
Since :: has a type for which no vector variant exists, it is useful to prevent a mixed list from being coerced into a vector when all items happen to be of the same type. (This is important when you need to preserve the ability to add non-conforming items later.)
q)x:(1;2;3)
q)x,:`a
'type
but
q)x:(::;1;2)
q)x,:`a / ok