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kdb

kdb how to pass a column name into a function

As a simplifying example, I have 

tbl:flip `sym`v1`v2!(`a`b`c`d; 50 280 1200 1800; 40 190 1300 1900)

and I d like to pass a column name into a function like

f:{[t;c];:update v3:2 * c from t;}

In this form it doesnt work. any suggestion how I can make this happen? Thanks

Solution

  • One option to achieve this is using @ amend:

    q){[t;c;n] @[t;n;:;2*t c]}[tbl;`v1;`v3]
sym v1   v2   v3
------------------
a   50   40   100
b   280  190  560
c   1200 1300 2400
d   1800 1900 3600

    This updates the column c in table t saving the new value as column n. You could also alter this to allow you to pass in custom functions too:

    {[t;c;n;f] @[t;n;:;f t c]}[tbl;`v1;`v3;{2*x}]