问使用for循环仅获取选定变量的相关性

EN

set.seed(1)
TDT <- data.table(Group = c(rep("A",40),rep("B",60)),
                      Id = c(rep(1,20),rep(2,20),rep(3,20),rep(4,20),rep(5,20)),
                      Time = rep(seq(as.Date("2010-01-03"), length=20, by="1 month") - 1,5),
                      norm = round(runif(100)/10,2),
                      y = sample(100,100),
                      x2 = round(rnorm(100,0.75,0.3),2),
                      x3 = round(rnorm(100,0.75,0.3),2),
                      x4 = round(rnorm(100,0.75,0.3),2),
                      x5 = round(rnorm(100,0.75,0.3),2))

# getting the numerical variables
numcols <- names(Filter(is.numeric,TDT))

# getting the correlation of y with all x
TDT.y.cor<- TDT[, cor(.SD, y),.SDcols = numcols, by = Time]

# getting the correlation of selected x with all other x
xcor <- c("x2","x3")
TDT.x.cor <- TDT[, cor(.SD, xcor),.SDcols = numcols, by = Time]

Error in cor(.SD, xcor) : 
  'y' must be numeric

xcor <- list ("x2","x3")
for (i in xcor) {
TDT.xcor[[i]].cor <- TDT[, cor(.SD, xcor[[i]]),.SDcols = numcols, by = Time]
}