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

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1

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 empy table tbl:flip refstampnoteusr!(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

2

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 “+ab!(`long$();())”

 

 

3

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

4

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!

5

I AM NOT WORTHY!!!

Great to hear from you David!

Thanks to you and rocuinneagain.

I appreciate both of you taking the time.

Both of you have a eGuinness on me.

  (eDrinks: like real ones but rubbish)

 

Simon