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high dimensional matrix index
apologies if this is obvious for Q professionals, I’m struggling to understand why running below in console returns 1 2 3 4 5 6, not 5 6 ? even ChatGPT says it returns 5 6…
mm:((1 2;3 4;5 6);(10 20;30 40;50 60))
mm 0 2
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4 Replies
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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
Q for Mortals: a textbook for kdb+ and the q programming language
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Thanks! good point to know!
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This reply was modified 2 weeks, 2 days ago by
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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/
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Thanks! maybe time to refresh my memory of the course did some time ago…
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