Get range of elements in KDB using variables

Question

Why I can't use variable inside array ranges in KDB?

test:1 2 3 4 5

This example won't work:

pos:3;
test[1 pos]

but this way it will work

test[1 3]

It doesn't work with semi-colon also. it says: 'type (wrong type) — user3914448, Feb 18 '15 at 08:29
See http://stackoverflow.com/questions/28525768/how-to-get-range-of-elements-in-a-list-in-kdb — Manish Patel, Feb 18 '15 at 09:12
It doesn't help as it doesn't use variables there also variable with sublist doesn't work also. — user3914448, Feb 18 '15 at 09:26
It's a simple extrapolation. `a+til b` gives you the list you require to get your range. `a` and `b` are variables. — Manish Patel, Feb 18 '15 at 09:41

score 1 · Accepted Answer · edited May 23 '17 at 12:06

As you can see, when you use test[1 3], (1 3) is a list. So vector variable requires a list.

   q) list1:1 3
   q) test[list1]

So you have to use:

   q)n:3
   q)list1:(1;n)
   q)test[list1]
   q)test[(1;n)]  / alternate way

For detail explanation about why only semicolon doesn't work and why we require brackets '()',check my answer for this post:

score 1 · Answer 2 · edited May 23 '17 at 12:06

To understand what you're asking, consider:

1 2 3 7

That is a simple list of integers. Now consider:

a 2 3

Where a is a vector. The above indexes into a. Easy. Now say you want to have that 2 3 list as a variable

b:2 3 a b //works!

You are specifically asking about how to get a range from a list, this is covered in How to get range of elements in a list in KDB?

In that answer, use variables to create your index list and use the result to index into a

2 Answers2