kdb how to calculate rolling count - kdb

Question

Assume I have a table of events, with Timestamp and Type.
t1, 'b'
t2, 'x'
t3, 's'
t4, 'b'
How can I get a rolling count such that it would give me a list of all timestamps and the cummulative number of events up to taht ts, sort of like a count version of sums
for example for 'b' I d like a table
't1', 1
't2', 1
't3', 1
't4', 2

Answer 1

Here is one way to do it, although there may be a more clever way this uses sums:
//table definition
tab:([]a:`t1`t2`t3`t4;b:"bxsb")
//rolling sum of 1 by column b
update sums count[i]#1 by b from tab
Results in:
a b x
------
t1 b 1
t2 x 1
t3 s 1
t4 b 2
If you wanted replace b you would simply put b: in front of the sums .

Answer 2

One way:
q)t:([]p:asc 4?.z.p+til 1000;t:`b`x`s`b)
q)asc `p xcols ungroup select p,til count i by t from t
p t x
---------------------------------
2017.05.16D09:42:48.259062090 b 0
2017.05.16D09:42:48.259062585 x 0
2017.05.16D09:42:48.259062683 s 0
2017.05.16D09:42:48.259062858 b 1
Ps: Note I have started the sequence at 0 as if to say "I've had 0 events prior to this row" instead of beginning at 1 as per your example. It goes with your req "number of events up to that ts". If you need 1, just add 1 '1+til count i'. Also ensure your time is sorted so as it makes sense when beginning the sequence.

Answer 3

With table t as below:
q)show t: ([]ts:.z.t - desc "u"$(til 4);symb:`b`x`z`b)
ts symb
-----------------
09:46:56.384 b
09:47:56.384 x
09:48:56.384 z
09:49:56.384 b
using a vector conditional:
q)select ts, cum_count:sums ?[symb=`b;1;0] from t
ts cum_count
----------------------
09:46:56.384 1
09:47:56.384 1
09:48:56.384 1
09:49:56.384 2
The same, but with a function taking symb as a parameter:
q){select ts, cum_count:sums ?[symb=x;1;0] from t}[`b]
ts cum_count
----------------------
09:46:56.384 1
09:47:56.384 1
09:48:56.384 1
09:49:56.384 2
In fact you don't need a vector conditional because you can just sum the booleans directly:
q){select ts, cum_count:sums symb=x from t}[`b]
ts cum_count
----------------------
09:46:56.384 1
09:47:56.384 1
09:48:56.384 1
09:49:56.384 2

Answer 4

This also works
update x:1+til count i by b from tab