high dimensional matrix index

[unterrainer_ale]

When indexing at depth you need the square brackets or index-at for nested indexing dot-apply

mm:((1 2;3 4;5 6);(10 20;30 40;50 60))
mm 0 2
1 2 3 4 5 6

0N!mm 0 2
((1 2;3 4;5 6);(`long$();`long$();`long$()))
1 2 3 4 5 6

mm[0;2]
5 6
mm . (0;2)
5 6
mm . 0 2

5 6   

If you really wanted you could use simple apply and the iterator over to achieve what you are looking for

mm@/0 2
5 6

If you use 0N! you can see that when using mm 0 2 you’re basically indexing at the first level, first for index 0 which returns the first top level, then at index 2 which doesn’t exist hence you get a list of empty long lists
3. Lists – Q for Mortals
Apply (At), Index (At), Trap (At) | Reference | kdb+ and q documentation – kdb+ and q documentation

• This reply was modified 2 weeks, 2 days ago by  unterrainer_ale.

code.kx.com

3. Lists - Q for Mortals

Q for Mortals: a textbook for kdb+ and the q programming language

[lestat-jin]

Thanks!  good point to know!

[mwoods]

To return 5 6 you will need to index at depth by passing a semicolon

mm[0;2]
5 6

Indexing by passing a list of integers returns all indexes so in this case:

mm 0 // returns the element at position 0
1 2
3 4
5 6

mm 2 // returns the element at position 2 - which doesn't exist
    

mm 0 2 // get both elements at positions 0 and 2 - because none at 2 we just get the 0 elements back 
1 2 3 4 5 6
  

This concept covered in the free academy course in the module on List Amendment and eliding at depth.

https://learninghub.kx.com/courses/kdb-developer-level-2/lessons/lists/topic/list-indexing-access/

[lestat-jin]

Thanks! maybe time to refresh my memory of the course did some time ago…