• kdb+

  formatting a table with @ (apply) to use lists as a format type (for example symbol "s" is "S")

  Posted by simon_watson_sj on December 21, 2022 at 12:00 am

  Hi Team,

  I’m wondering how to use the @ (apply) paradigm to  set a table column so that it accepts lists rather than atoms.

  The below illustrates my problem. I can use @ (apply) to cast a column to an atomic data type but can’t seem to manage to get it to a listed datatype.

   

  // define an empty table 
  tbl:flip `ref`stamp`note`usr!(`symbol$();`timestamp$();"";()) 
  
  //use apply to make column usr a symbol type: 
  @[tbl;`usr;{y$x};"s"] // works (test using meta tbl) 
  
  //use apply to make column note a string type (nested character): 
  @[tbl;`usr;{y$x};"C"] // does not work - result is a column of type c (single character) (test using meta tbl)

   

  My question is:

  Is there a way to use apply (@) to set a column in a table to a nested data type (so “C” for a string or “S” for a list of symbols for each record)?

  Regards,

  Simon

  simon_watson_sj replied 10 months, 1 week ago 3 Members · 3 Replies
• 3 Replies

• 

  rocuinneagain

  Member

  December 21, 2022 at 12:00 am

  meta on an empty table will never show C. This is because kdb+ does not have a dedicated datatype for lists of lists. The empty list will be () which is of type 0h. On a populated table meta inspects the first item in a list of lists and uses it’s type to populate it’s result. It cannot do this on an empty table.

   

  q)tb:([] a:1 2;b:("wo";"rd")) 
  q)tb 
  a b 
  ------ 
  1 "wo" 
  2 "rd" 
  
  q)meta tb 
  c| t f a 
  -| ----- 
  a| j 
  b| C 
  
  q)delete from `tb 
  `tb 
  
  q)tb 
  a b 
  --- 
  
  q)meta tb 
  c| t f a 
  -| ----- 
  a| j 
  b| 
  
  q).Q.s1 tb 
  "+`a`b!(`long$();())"

   

   

• 

  davidcrossey

  Member

  December 22, 2022 at 12:00 am

  Hi Simon,

  You can leave the type of the (future) string column as empty in the table definition and let q infer the type once the first upsert occurs:

   

  q)test: ([]test: `$(); uptar:()); 
  q)test upsert ([]test: `rta; uptar: enlist "ra") 
  test uptar 
  ---------- 
  rta "ra" 
  
  q)meta test upsert ([]test: `rta; uptar: enlist "ra") 
  c    | t f a 
  -----| ----- 
  test | s 
  uptar| C

   

  Happy Christmas!

• 

  simon_watson_sj

  Member

  December 22, 2022 at 12:00 am

  Thanks rocuinneagain – that’s good insight.

  Given the above constraint, I have the following issue:

  // define a table with the intention of loading a string to a columnn.

  // as discussed, it is empty so ‘uptar’ is defined as having type “c”.

  test: ([]test: `$(); uptar:"C"$());

  // With this done, I can’t see a valid upsert to get a string into the uptar column.

  test upsert ([]test: `rta; uptar: enlist "ra"); 
  test upsert ([]test: enlist `rta; uptar: "ra");
  test upsert ([]test: enlist `rta; uptar:enlist enlist "ra");

  // finally, this one projects – so works but still not correctly.

  test upsert ([]test: `rta; uptar: "ra");

   

  Is it possible to insert a nested list into an empty table or must we use a ‘set’ workaround?

  Thanks and regards,

  Simon

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