replace looped subsetVector with subsetDT (#5264)

R/data.table.R

@@ -1303,18 +1303,8 @@ replace_dot_alias = function(e) {
         #       But rather than that complex logic here at R level to catch that and do a shallow copy for efficiency, just do the check inside CsubsetDT
         #       to see if it passed 1:nrow(x) and then CsubsetDT should do the shallow copy safely and centrally.
         #       That R level branch was taken out in PR #3213
-
-        # TO DO: use CsubsetDT twice here and then remove this entire R level branch
-        for (s in seq_along(icols)) {
-          target = icolsAns[s]
-          source = icols[s]
-          ans[[target]] = .Call(CsubsetVector,i[[source]],ii)  # i.e. i[[source]][ii]
-        }
-        for (s in seq_along(xcols)) {
-          target = xcolsAns[s]
-          source = xcols[s]
-          ans[[target]] = .Call(CsubsetVector,x[[source]],irows)   # i.e. x[[source]][irows], but guaranteed new memory even for singleton logicals from R 3.1.0
-        }
+        ans[icolsAns] = .Call(CsubsetDT, i, ii,    icols)
+        ans[xcolsAns] = .Call(CsubsetDT, x, irows, xcols)
         setattr(ans, "names", ansvars)
         if (haskey(x)) {
           keylen = which.first(!key(x) %chin% ansvars)-1L