rbind two data.frame preserving row order and row names

Question

I have a list of data.frame objects which i would like to row append to one another, ie merge(..., all=T). However, merge seems to remove

Answer 1

Since you know you are not actually merging, but just rbind-ing, maybe something like this will work. It makes use of rbind.fill from "plyr". To use it, specify a list of the data.frames you want to rbind.

RBIND <- function(datalist) {
  require(plyr)
  temp <- rbind.fill(datalist)
  rownames(temp) <- unlist(lapply(datalist, row.names))
  temp
}
RBIND(list(x, y))
#               a  b  c  d
# row_1         1  2  3  4
# another_row1  2  3  4  5
# row_2        10 20 30 NA
# another_row2 20 30 40 NA

Answer 2

One way is to use row.names in merge so that you get it as an additional column.

> merge(x, y, by=c("row.names", "a","b","c"), all.x=T, all.y=T, sort=F)

#      Row.names  a  b  c  d
# 1        row_1  1  2  3  4
# 2 another_row1  2  3  4  5
# 3        row_2 10 20 30 NA
# 4 another_row2 20 30 40 NA

Edit: By looking at the merge function with getS3method('merge', 'data.frame'), the row.names are clearly set to NULL (it is a pretty long code, so I won't paste here).

# Commenting 
# Lines 63 and 64
row.names(x) <- NULL
row.names(y) <- NULL

# and 
# Line 141 (thanks Ananda for pointing out)
attr(res, "row.names") <- .set_row_names(nrow(res))

and creating a new function, say, MERGE, works as the OP intends for this example. Just an experimentation.