call function with multiple value kdb\q

Question

I have a function declaration:

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

where instr_id has type i and tp_time has type v.

For example, func[8;05:00:11] works fine and gives me value 17.55.

However, if I try func[8 1;05:00:11 07:10:59], I get:

'length ERROR: incompatible lengths (different lengths of operands for synchronized operation or table columns lengths are not the same

What I want to get is, for example, 17.55 9.66.

Same error will pop up as well if I do select res:func_demo[id;time]from tab, where tab is table with two column of instr_id and tp_time.

I guess enlist could possibly resolve the problem, but I don't know exactly how to use it. How could I fix this issue?

Answer 1

Both of the answers here are great but don't give much detail about why you got the 'length error. The error was all down to your where clause where instr_id = id, tp_time < time. When you passed a list of items you actually created a list of boolean lists that are being passed to the where clause. Setting up two example lists to highlight this:

q)show instr_id:-5?10
2 8 0 1 6
q)show tp_time:5?.z.t
01:05:42.696 03:42:39.320 17:44:49.101 15:01:01.470 05:47:49.777

Running your first condition instr_id = id with atoms an lists:

q)instr_id=2
10000b
q)instr_id=2 8
'length
  [0]  instr_id=2 8
               ^

Using = will only work for an atom of a list of equal length to instr_id. If you want to pass multiple items to match you can instead use in:

q)instr_id in 2
10000b
q)instr_id in 2 8
11000b

For the second condition tp_time < time again requires atoms or lists of the same length to be passed:

q)tp_time<.z.t
11111b
q)tp_time<.z.t,.z.t
'length
  [0]  tp_time<.z.t,.z.t
              ^

Can get around this by making use of adverbs, such as each-right /: to compare tp_time to multiple items and using any to cut it down to a single list:

q)tp_time</:.z.t,.z.t
11111b
11111b
q)any tp_time</:.z.t,.z.t
11111b

Hope this helps you to understand where you ran into issues above.