Re: How can i understand the code below

[eohara]

Hi,

 

The : in :@ is an explicit return (see Function notation Basics kdb+ and q documentation – Kdb+ and q documentation (kx.com)).

The @ is an amend operator (see Amend, Amend At Reference kdb+ and q documentation – Kdb+ and q documentation (kx.com)).

`p# is the parted attribute (see Tables | Q For Mortals | A textbook forkdb+ and the q programming language – Q for Mortals (kx.com)).

` is the first argument of sv here as we wish to build a filepath to a splayed table (see sv scalar from vector | Reference | kdb+ and q documentation – Kdb+ and q documentation (kx.com)).

To bring that altogether, what is happening in your function is:

1. get table
   1. This returns the table data for the given table name
2. .Q.en[tabPath] get table
   1. This enumerates the output of 1) against a sym file in “tabPath”, and returns the enumerated table
3. (`  sv (tabPath; ` ; table; ` ) ) set .Q.en[tabPath] get table
   1. Writes the table from 2) to the filepath, and returns the filepath if successful e.g. if tabPath is `:/home/kx/db and table is `trade, then (`  sv (tabPath; ` ; table; ` ) ) will return `:/home/kx/db/trade/
      1. The trailing slash after trade means the table will be saved splayed
4. pvar xasc (`  sv (tabPath; ` ; table; ` ) ) set .Q.en[tabPath] get table
   1. Sorts the written table in 3) on the column in the pvar variable
5. :@[;pvar; `p#] pvar xasc (`  sv (tabPath; ` ; table; ` ) ) set .Q.en[tabPath] get table
   1. Amends the sorted table on disk, adding a parted attribute to the column in the pvar variable