kdb: Add a column showing sum of rows in a table with dynamic headers while ignoring nulls - sql

Question

I have a table whose columns are dynamic, except one column:A. The table also has some null values (0n) in it. How do I add another column that shows total of each row and either ignores the column that has "0n" in that particular row or takes 0 in its place.
Here is my code, it fails on sum and also does not ignore nulls.
addTotalCol:{[]
table:flip`A`B`C`D!4 4#til 9;
colsToSum: string (cols table) except `A; / don't sum A
table: update Total: sum (colsToSum) from table; / type error here. Also check for nulls
:table;
}

Answer 1

I think it is better to use functional update in your case:
addTotalCol:{[]
table:flip`A`B`C`D!4 4#til 9;
colsToSum:cols[table] except `A; / don't sum A
table:![table;();0b;enlist[`Total]!enlist(sum;enlist,colsToSum)];
:table;
}
Reason why it is not working is because your fourth line is parsed as:
table: update Total: sum (enlist"B";enlist"C";enlist"D") from table;
Since sum only works with numbers, it returns 'type error since your inputs are string.
Another solution to use colsToSum as string input:
addTotalCol:{[]
table:flip`A`B`C`D!4 4#til 9;
colsToSum:string cols[table] except `A; / don't sum A
table:get"update Total:sum(",sv[";";colsToSum],") from table"
:table;
}
Basically this will build the query in string before it is executed in q.
Still, functional update is preferred though.
EDIT: Full answer to sum 0n:
addTotalCol:{[]
table:flip`A`B`C`D!4 4#0n,til 9;
colsToSum:cols[table] except `A; / don't sum A
table:![table;();0b;enlist[`Total]!enlist(sum;(^;0;enlist,colsToSum))];
:table;
}

Answer 2

I think there is a cleaner version here without a functional form.
q)//let us build a table where our first col is symbols and the rest are numerics,
/// we will exclude first from row sums
q)t:flip `c0`c1`c2`c3!(`a`b`c`d;1 2 3 0N;0n 4 5 6f;1 2 3 0Nh)
q)//columns for sum
q)sc:cols[t] except `c0
q)///now let us make sure we fill in each column with zero,
/// add across rows and append as a new column
q)show t1:t,'flip enlist[`sumRows]!enlist sum each flip 0^t sc
c0 c1 c2 c3 sumRows
-------------------
a 1 1 2
b 2 4 2 8
c 3 5 3 11
d 6 6
q)meta t1
c | t f a
-------| -----
c0 | s
c1 | i
c2 | f
c3 | h
sumRows| f