• kdb+

  Why do we use sel[x] here in u.q, could we not have just used x?

  Posted by mannix on March 17, 2022 at 12:00 am

  sel:{$[`~y;x;select from x where sym in y]} / from u.q

  My understanding is that sel returns x unless you enter a y?

  If this is the case, could we not have used “x” instead of “sel[x]” in pub?

  pub:{[t;x]{[t;x;w]if[count x:sel[x]w 1;(neg first w)(`upd;t;x)]}[t;x]each w t}

  mannix replied 8 months, 1 week ago 2 Members · 1 Reply
• 1 Reply

• 

  pmallon

  Member

  March 17, 2022 at 12:00 am

  Hi,

  The sel[x]w 1 , call is the same as sel[x;w 1].  So, y =w 1, which will be a list of symbols or ` if no filtering on syms is to be applied.
  Then within sel function, filtering is applied if y is not equal to `  with the select from x where sym in y.

  Omitting sel from the pub function would remove this functionality to subscribe to a subset of syms.

  Let me know if this helps or not.

  Patrick

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