Understand elided index

[dbtouch]

Hi, KDB Folks

I am having difficulty understanding elided index example in q
language reference manual:

d:((1 2 3;4 5 6 7);(8 9;10;11 12);(13 14;15 16 17 18;19 20))

what is the straight forward way to evaluate d[0 2;;1 0]?

Thanks

[Oleg Zakharov]

Meaning what exactly?

You can think of it as a projecting 3-dim array, you're projecting it to 4 lines

To comprehend better what will happen here, do it in steps

you have d (unfortunately console isn't 3d ;-))

So, you'll have elements listed vertically by top (leftmost) index. Upper is 0th 

q)d

(1 2 3;4 5 6 7)

(8 9;10;11 12)

(13 14;15 16 17 18;19 20)

d[0;;] (leaving only 0th element - flattening 3-dim to 2-dim, along 0 as a 1st index) 

q)d[0;;]

1 2 3

4 5 6 7

((1 2 3;4 5 6 7) that is)

d[0 2;;] (leave only 0th and 2nd elements toplevel-wise)

q)d[0 2;;]

(1 2 3;4 5 6 7)

(13 14;15 16 17 18;19 20)

Then, you are projecting this result further, now from the deep end

q)d[0 2;;][;;0]

1 4

13 15 19

meaning, that from the each of the 3rd level lists you want to keep only 0th element

or, as original design was, you want to leave 2 elements, 1st and 0th (in this order, reverse to original) 

q)d[0 2;;][;;1 0]

(2 1;5 4)

(14 13;16 15;20 19)

That's it. Compact notation is not affecting the result.

 q)d[0 2;;1 0]

(2 1;5 4)

(14 13;16 15;20 19)

As a side note - there's a great book on Q (best ever - as it's the only one ;-))

https://code.kx.com/trac/wiki/QforMortals2/contents

Free to read from kx site.

Please, consider reading it - lots of fun & time well invested.

Cheers,

Oleg

 

--
You received this message because you are subscribed to the Google Groups "Kdb+ Personal Developers" group.
To post to this group, send email to personal...@googlegroups.com.
To unsubscribe from this group, send email to personal-kdbpl...@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/personal-kdbplus?hl=en.

[dbtouch]

That is awesome. Thanks, Oleg.

> ;-))https://code.kx.com/trac/wiki/QforMortals2/contents