Using fby for cumulative sum in KDB/Q, but not getting the right output

I am trying to use the fby function to create a new column that cumulatively sums a Volume column by a Symbol.

I have a table named x that looks like this:

I tried the following fby function to do so, but it just spits out the same as the "VOL" column:

x: update cumSum: sums (deltas;VOL) fby Symbol from x

x

However, this is what I am expecting the following:

Please let me know if you know where I’m making a mistake or have some advice, appreciate the help.

>Solution :

deltas isn’t needed in your above example – below will accomplish what you’re looking to do

q)show x:([]Timestamp:"Cell ",/:"123456";Symbol:`A`A`A`B`B`B;VOL:2 5 2 3 6 1;Price:6#10 20)
Timestamp Symbol VOL Price
--------------------------
"Cell 1"  A      2   10
"Cell 2"  A      5   20
"Cell 3"  A      2   10
"Cell 4"  B      3   20
"Cell 5"  B      6   10
"Cell 6"  B      1   20
q)update cumSumVOL:(sums;VOL)fby Symbol from x
Timestamp Symbol VOL Price cumSumVOL
------------------------------------
"Cell 1"  A      2   10    2
"Cell 2"  A      5   20    7
"Cell 3"  A      2   10    9
"Cell 4"  B      3   20    3
"Cell 5"  B      6   10    9
"Cell 6"  B      1   20    10