Merging two data frames, but keeping NAs

Adding the argument all.x=TRUE to merge() will retain the NA
values, but the only reliable way I've found to preserve order with
NA values in a merge is to add an index column to x, merge the
data, sort on the index column, then delete it.

Sarah

On Thu, Dec 5, 2013 at 9:56 AM, Rainer M Krug <Rainer at krugs.de>
wrote:

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Hi

My brain is giving up on this...

I have the following two data.frames:

x <-  data.frame(ref=c(NA, NA, NA, 10:5, NA, 1:5)) y <-
data.frame(id = c(2, 3, 4, 6, 7, 9, 8), val = 101:107)

Which look as follow:

x

ref 1   NA 2   NA 3   NA 4   10 5    9 6    8 7    7 8    6 9
5 10  NA 11   1 12   2 13   3 14   4 15   5

y

id val 1  2 101 2  3 102 3  4 103 4  6 104 5  7 105 6  9 106 7  8
107

Now I want to merge y into x, but that

a) the sort order of x stays the same (sort=FALSE in merge())
and b) the NAs stay

The result should look as follow (column id only here for
clarity):

result

ref  id  val 1   NA  NA  NA 2   NA  NA  NA 3   NA  NA  NA 4   10
NA  NA 5    9   9   106 6    8   8   107 7    7   7   105 8    6
6   104 9    5  NA  NA 10  NA  NA  NA 11   1  NA  NA 12   2   2
101 13   3   3  102 14   4   4  103 15   5  NA  NA

merge(x, y, by.x="ref", by.y="id", sort=FALSE) leaves out the NA,
but otherwise it works:

merge(x, y, by.x=1, by.y="id", sort=FALSE)

ref val 1   9 106 2   8 107 3   7 105 4   6 104 5   2 101 6   3
102 7   4 103

Is there any way that I can tell merge() to keep the NA, or how
can I achieve what I want?

Thanks,

Rainer