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Accessing previous calculated value from a column in current row
Hello q experts,
I have a table created like below.
t:([]c1: 1 2 3 4 5 6 7 8 9)
Below statement works fine as I am using previous value of column c1.
update c2:((prev(c1)+10) + c1)%2 from t
If I try to use previous value from column c2, then there is a error.
update c2:((prev(c2)+10) + c1)%2 from t
How can I use the previous calculated value of c2 in the current row calculation?
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6 Replies
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https://code.kx.com/q/ref/accumulators/#binary-values
q)select c1,c2:0{(x+10+y)%2}c1 from t c1 c2 ----------- 1 5.5 2 8.75 3 10.875 4 12.4375 5 13.71875 6 14.85938 7 15.92969 8 16.96484 9 17.98242 -
Hello,To modify the table in place, pass it by name (i.e. `t).
q)t:([]c1: 1 2 3 4 5 6 7 8 9) q)update c2:((prev(c1)+10) + c1)%2 from `t `t q)update c2:((prev(c2)+10) + c1)%2 from `t `t q)t c1 c2 -------- 1 2 3 9.75 4 10.75 5 11.75 6 12.75 7 13.75 8 14.75 9 15.75
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Thanks for the reply vkennedy.
Actually I don’t want to update the existing table at all. Sorry for confusing with update keyword. (But I learnt something new here Thanks).
Please consider below case. The select fails here.
t:([]c1: 1 2 3 4 5 6 7 8 9)select c1, c2:((prev(c2)+10) + c1)%2 from t
How can I use the previous calculated value of c2 in the current row calculation? For the first row in the table there is no prev(c2); so that could be considered as 0. From the second row, it should consider the previous calculated value of c2.
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Hi,
Is this the result you are looking for?
q)select c2:(0^(prev(c2)+10) + c1)%2 from update c2:((0^prev(c1)+10) + c1)%2 from t c2 ----- 0 6.25 9.75 10.75 11.75 12.75 13.75 14.75 15.75
In this case, you need to explicitly fill in the zero as the result of prev on the first item of a list is null.
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Hi,
I have added the calculations below.
c1 c2 Calculation 1 5.5 =((0+10) + 1)/2 =((prev(c2)+10)+c1)/2 2 8.75 =((5.5+10) + 2)/2 3 10.875 =((8.75+10) + 3)/2 4 12.4375 =((10.875+10) + 4)/2 5 13.71875 =((12.4375+10) + 5)/2 6 14.859375 =((13.71875+10) + 6)/2 7 15.9296875 =((14.859375+10) + 7)/2 8 16.96484375 =((15.9296875+10) + 8)/2 c2 column values is the expected result.
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Wow!! Thanks Jason & vkennedy.
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