从data.table的`j`内部访问`by`变量 [英] Access `by` variable from within `j` in data.table
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问题描述
如果我有一个 data.table
并且我正在 j
中进行一些函数调用,我是否可以可以访问 by
变量的当前值吗?
If I have a data.table
and I'm doing some function call in the j
, do I have access to the current value of the by
variable?
library(data.table)
d <- data.table(x=1:10, y=c('a', 'b'))
myfun <- function(DT) {
print (DT$y)
}
d[, myfun(.SD), by=y]
有关更多上下文,我将第二个参数(另一个 data.table
)传递给 myfun
,并希望基于 y
的当前值对其进行子集化。可以用一个虚拟变量来完成,但这似乎令人讨厌...
For more context, I am passing a second argument (another data.table
) to myfun
and want to subset that based on the current value of y
. It could be done with a dummy variable, but that seems yucky...
推荐答案
使用 .BY
-这是 by
变量的列表:
Use .BY
- which is a list of by
variables:
d <- data.table(x=1:10, y=c('a', 'b'))
d[, .BY[[1]], by = y] # [[1]] to access the first by variable, which is y
# if you had by = list(x, y) you'd do .BY[[2]] to access y
# y V1
#1: a a
#2: b b
此外,列表为
d[, .BY$y, by = y]
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