Merge multiple data tables with duplicate column names

Question

I am trying to merge (join) multiple data tables (obtained with fread from 5 csv files) to form a single data table. I get an error when I try to merge 5 data tables, but wo

Answer 1

stack and reshape I don't think this maps exactly to the merge function but...

mycols <- "x"
DTlist <- list(DT1,DT2,DT3,DT4,DT5)

dcast(rbindlist(DTlist,idcol=TRUE), paste0(paste0(mycols,collapse="+"),"~.id"))

#    x  1  2  3  4  5
# 1: a 10 11 12 13 14
# 2: b 11 12 13 14 15
# 3: c 12 13 14 15 16
# 4: d 13 14 15 16 17
# 5: e 14 15 16 17 18
# 6: f 15 16 17 18 19

I have no sense for if this would extend to having more columns than y.

merge-assign

DT <- Reduce(function(...) merge(..., all = TRUE, by = mycols), 
  lapply(DTlist,`[.noquote`,mycols))

for (k in seq_along(DTlist)){
  js = setdiff( names(DTlist[[k]]), mycols )
  DT[DTlist[[k]], paste0(js,".",k) := mget(paste0("i.",js)), on=mycols, by=.EACHI]
}

#    x y.1 y.2 y.3 y.4 y.5
# 1: a  10  11  12  13  14
# 2: b  11  12  13  14  15
# 3: c  12  13  14  15  16
# 4: d  13  14  15  16  17
# 5: e  14  15  16  17  18
# 6: f  15  16  17  18  19

(I'm not sure if this fully extends to other cases. Hard to say because the OP's example really doesn't demand the full functionality of merge. In the OP's case, with mycols="x" and x being the same across all DT*, obviously a merge is inappropriate, as mentioned by @eddi. The general problem is interesting, though, so that's what I'm trying to attack here.)