How to remove element from list in KDB?

Question

For instance, I have:

x: (`a`b!(1;2); (); `a`b!(3;4))

and I want to remove the (). I have tried using ? (match)

x[x ? not ()]

but that just gives ().

What is the correct expression?

Background

I often use peach to execute a series of queries, and often some will return missing data in the form of (). Hence I want to remove the () and get back to a nice table.

Answer 1

You can use except as well:

q)x except 1#()
a b
---
1 2
3 4

Answer 2

Well, if you are returning tables already (rather than dictionaries), you could always just raze and that would be enough to get rid of the empty list. Technically, it is appended, but if you append an empty list on a list, you get the original list. Recall that a table is simply a list of dictionaries, so this remains the case here as well.

//I'm enlisting your dictionaries,
//so that they become tables and then raze
q)raze @[x; 0 2; enlist]   
a b
---
1 2
3 4

Snippets such as raze f each x or raze f peach x are very common, and I suspect this is the idiom that would be best for your use case.

Answer 3

() is an empty list. So it has count of zero. Therefore we can use:

q)x where not 0 = count each x
a b
---
1 2
3 4

Answer 4

A slight more generic way to get rid of unwanted elements :

q)x where not x~\:()
a b
---
1 2
3 4

q)x where not x~\:`a`b!(1;2)
()
`a`b!3 4