数据表| .SD lapply '...'-参数中的多列

我如何使用固定的特定列...的论证lapply(.SD, FUN, ...)

Example

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]

结果是

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

一个可能的解决方法是

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？没有参考y by y=DT[,id_column]

Issue #495 https://github.com/Rdatatable/data.table/issues/495现在解决了最近的这个提交 https://github.com/Rdatatable/data.table/commit/68091d851d400b4d768f3c41c0210da76945c12b，我们现在可以很好地做到这一点：

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