[R] merging issue.........

[karena]

hi, I have a question about merging two files.
For example, I have two files, the first file is like the following:

id   trait1
1    10.2
2    11.1
3    9.7
6    10.2
7    8.9
10  9.7
11  10.2

The second file is like the following:
id    trait2
1     9.8
2     10.8
4     7.8
5     9.8
6     10.1
12    10.2
13    10.1

now I want to merge the two files by the variable "id", I only want to keep
the "id"s which show up in the first file. Even the "id" does not show up in
the second file, it doesn't matter, I can keep the missing values. So my
question is: how can I merge the two files and keep only the rows whose "id"
show up in the first file?
I know how to do it is SAS, just use the following code: 
merge data1(in=in1) data2(in=in2);
by id;
if in1;

but I really have no idea about how to do it in R.

thank you in advance,

karean 
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