function return a table in kdb/q

I'm new in kdb/q and I'm not familiar with kdbq function, hope someone can help me. Here is the question:
I have a simple q function declared as the following:

func:{[x;y] x+y}


And {[x;y] x+y}[3;4] gives me the answer 7. Everything works perfectly.

{[x;y] x+y}[3;4]

7

If I have a table t with two columns such as:

t

_x _y
--------
3 4
2 5
6 2
...


Could I have a function in q such that compute x+y for each row of tablet?
And my expected return would be something like:

x+y

t

res
---
7
7
8
...


Thanks so much!

2 Answers
2

You can just pass the column names as parameters to the function:

q)tab:(x:1 2 3;y:4 5 6)
q)func:{[x;y] x+y}
q)
q)select res:func[x;y]from tab
res
---
5
7
9


Alternatively you could use functional form to turn that query into a function:

q){?[x;();0b;enlist[`res]!enlist(`func;y;z)]}[tab;`x;`y]
res
---
5
7
9


That's aweson! Could you explain the second alternative to me>? I don't quite get it..
– Chenrui Su
Jun 29 at 15:59

Its working with the functional form of selects, which is described in decent detail on the wiki. To see what functional form looks like for a query you can run parse"select res:func[x;y]from tab" which will output the 4 parameters required to make up the query.
– Thomas Smyth
Jun 29 at 16:03

parse"select res:func[x;y]from tab"

Thanks so much! One more quick question: i have a function func:{[id;time] select last synp from synp_t where instr_id = id, tp_time < time} ; but when I run func[8 1;05:01:06 05:06:01], it gives me 'length ERROR.. do you know where I was wrong..?
– Chenrui Su
Jun 29 at 16:07

func:{[id;time] select last synp from synp_t where instr_id = id, tp_time < time}

func[8 1;05:01:06 05:06:01]

Since + is overloaded to work with both atom and list , res:func[x;y] will work perfectly fine; however in cases when a dyadic function only accepts the arguments as atoms rather than lists then each-both will do the trick:

+

res:func[x;y]

each-both

q)select res:func'[x;y] from tab // using each-both func'[x;y]
res
---
5
7
9


e.g To select as many charecters as c from column s

c

s

tab2:( c:1 2 3;s:("123";"1234";"123456"))
q)update res:#'[c;s] from tab2 //func'[x;y]
c s res
-----------------------
1 "123" enlist "1"
2 "1234" "12"
3 "123456" "123"


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