[S] Problem with Merge

A side-by-side, or relational, merge of two data frames with exactly
the same set of variables doesn't make any sense. Perhaps you wanted
an end-to-end concatenation of them, that is, have a new data frame
with the same variables and all the rows from both. For that, use
rbind().
In your example, rbind(xa[,1:2],xb[,1:2])

-Don

I want to merge two data frames that have the same set of variable
names. The docs on merge don't say this isn't possible but it
doesn't appear to be possible.

 xa

  a  b c1
1 1 10  1
2 2 11  2
3 3 12  3

 xb

  a  b c2
1 3 12  1
2 4 14  2
3 5 15  3
4 6 16  5

 merge(xa,xb,all=T)

  a  b c1 c2
1 1 10  1 NA
2 2 11  2 NA
3 3 12  3  1
4 4 14 NA  2
5 5 15 NA  3
6 6 16 NA  5

 merge(xa[,1:2],xb[,1:2],all=T)  # Only the identical vector names

Error in "names<-.default"(*tmp*, value = vnames) :
 names attribute must be the same length as the vector

merge(xa,xb[,1:2],all=T) # Only one data frame has a nonmatching name

Error: attempt to select less than one element

It appears that for merge to work, there must be at least one
variable in each data frame that does not match. So in order to
merge two data frames with the same set of
names (something that would seem to be frequently needed), you have
to add one dummy vector with different names to each data frame.

The docs for merge say that if you don't specify a "by" variable,
all variables in common are used. I don't see it mentioned that you
can't have all variables in
common.

Rick Bilonick

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Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
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