I sometimes want to check the number of rows for each value of a variable.
I use the following method. The first syntax doesn't work, but the second
does. I think that they ought to give the same result.
> DT <- data.table(x=rep(c(1, 2), each=10), y=1:20)
> DT[, length(x), list(x)]
x V1
[1,] 1 1
[2,] 2 1
> DT[, length(y), list(x)]
x V1
[1,] 1 10
[2,] 2 10
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