Set string data type

[Hemant Verma]

Hi,

I need to set the data type for columns c3,c4 as "C" without inserting single record in table

suppose, tab is table having c3,c4 is list column
tab:([] c1:`int$(); c2:`boolean$(); c3:(); c4:())
q)meta tab    
c | t f a     
--| -----
c1| i    
c2| b   
c3|     
c4|        

I want to set c3 and c4 as "C" (string data type) with inserting a record
q)meta tab    
c | t f a    
--| -----
c1| i   
c2| b   
c3| C   
c4| C            

Please let me know how can I do this.

Thanks,

Hemant

[Hemant Verma]

Correction: I need to do without inserting the record.

[Jack Andrews]

This came up before and I think the answer was "can't be done".

There is no type for "list of strings" in q, well, there is, but it's 0h, the same as for any list of lists.

kdb+ can only decide the type of generic columns after the first row is inserted.

but even then, there are no checks that the future inserts match the type of the first row:

q)p:([]a:`long$();c:())

q)`p insert (3;1 2)

,0

q)p

a c

-----

3 1 2

q)`p insert (3;"df")

,1

q)p

a c

-----

3 1 2

3 d f

q)`p insert (3;"dfd")

,2

q)p

a c

-------

3 1 2

3 "df"

3 "dfd"

q)meta p

c| t f a

-| -----

a| j

c| J

ta, jack

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[RAHUL ASATI]

Hi,

No you  can not do that. In case of table column types,the capital letters that are given by meta  like "I", "C" ,"J"  etc are not an actual datatype in kdb. They are just used to represent general list with some additional characteristics in case of tables. So if you want to convert empty list () to "C" (or "I"), basically its the conversion from general list to general list. Its only after first record you can get "C" but again kdb doesn't stops you from inserting any other type of data later in that column as also mentioned in other post. 

Example:

q) t:([]a:(1 2;3 4))

q) meta t

q) c | t f a

    a | J

q) type t`a

 q) 0h