[R] how to emerge two tables by taking the ave.

[affy snp]

Dear list,

I am new to R and very inexperienced. Sorry for the trouble.
I have two txt files and want to merge them by taking the average.
More specifically, for example, the txt file1, with row names and column names,
consists of 238000 rows and 196 columns. Each column corresponds
to a sample. The data is mixed with numeric or NA. So what I plan to
do is:

(1) Take the 1st column from txt file 1 and txt file 2, calculate the average
if both numbers are numeric. If one is numeric and the other one is NA or
the opposite, just use the numeric; If both are NA, then use NA, Do all this
for all columns
(2) Create txt file 3 with the numbers from the above and add the row names and
column names.

So an illustrative example could be:

txt file 1

             A       B       C
row1      2       4        6
row2      1       2       NA
row3      4       6       NA

txt file 2

            A        B      C
row1     NA      4       4
row2     2         2      NA
row3     1         2      2

then txt file 3 will be created as:

            A        B       C
row1     2         4       5
row2    1.5       2       NA
row3    2.5       4       2

Any help will be appreciated.

Thanks!
       Allen