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Editorial Team
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Asked: 2026-06-15T06:48:53+00:00

I have a data.table with one key and about 100 numeric rows, one of

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I have a data.table with one key and about 100 numeric rows, one of which is set to key. I would like to create a new variable that contains summation of each numeric rows, grouped by key.

For example, my data right now is

ID Count1 Count2 Count3
1   1      3      0
1   3      3      3
2   1      2      1
3   1      1      2

What I would like to have is:

ID Count1 Count2 Count3
1   4      6      3
2   1      2      1
3   1      1      2

I have tried so many ways to get this. I know I can do:

Y <- X[, list(Count=sum(Count1), Count2=sum(Count2), Count3=sum(Count3), by = ID]

However, I have hundreds of variables, and I only get their names on a list. How should I go about handling this?

Thanks a lot for your help.

Here is a code to generate test data:

ID <-c(rep(210, 9), rep(3917,6))
Count1 <- c(1,1,0,1,3,1,4,1,1,1,1,1,1,0,1)
Count2 <- c(1,0,0,1,0,1,0,1,1,1,1,1,1,0,1)
Count3 <- c(1,0,0,1,0,1,0,1,1,1,1,1,1,0,1)
x <- data.table(ID, Count1, Count2, Count3)
setkey(x, ID)
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1 Answer

  1. Editorial Team

    Your test data doesn’t match the example you gave, but regardless – you can take advantage of the fact that data.table() has an operator named .SD for “subset of data. So this should work:

    x[, lapply(.SD, sum), by = ID]
#----
     ID Count Count2 Count3
1:  210    13      5      5
2: 3917     5      5      5

    This is actually covered in the FAQ: type vignette("datatable-faq", package="data.table") or find it online.

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