data.table | .SD在`...`-Argument中加载多个列 [英] data.table | .SD lapply multiple columns in `...`-Argument

问题描述

如何使用固定的特定列 ... 参数 lapply（.SD，FUN，...）

How can i use a fixed specific column the ... argument of lapply(.SD, FUN, ...)

示例

DT <- data.table(id_column = rnorm(10),
                 x1 = rnorm(10), x2 = rnorm(10), x3 = rnorm(10))
measure_col = paste0("x",1:3)

DT[,lapply(.SD, cov, y=id_column), .SDcols = measure_col]

结果

Error in is.data.frame(y) : object 'id_column' not found

可能的解决方法是

DT[,lapply(.SD, cov, y = DT[,id_column]), .SDcols = measure_col]
          x1         x2        x3
1: 0.1703253 -0.2831533 0.3387133

有更好的方法吗？不通过 y = DT [，id_column]

Is there a better way of doing it? Without referencing to y by y=DT[,id_column]

推荐答案

问题＃495 现在解决了此最近提交，我们现在可以这样做：

Issue #495 is solved now with this recent commit, we can now do this just fine:

require(data.table) # v1.9.7+
DT <- data.table(id_column = rnorm(10), x1 = rnorm(10), x2 = rnorm(10), x3 = rnorm(10))
measure_col = paste0("x",1:3)    
DT[,lapply(.SD, cov, y=id_column), .SDcols = measure_col]
#             x1        x2         x3
# 1: -0.03137294 0.1903654 -0.1493648

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