'merge' function: behavior w.r.t. NAs in the key column

On Fri, 14 Mar 2008, Simon Anders wrote:

I recently ran into a problem with 'merge' that stems from the way how
missing values in the key column (i.e., the column specified
in the "by" argument) are handled. I wonder whether the current behavior
is fully consistent.
...

x <- data.frame( key = c(1:3,3,NA,NA), val = 10+1:6 )
y <- data.frame( key = c(NA,2:5,3,NA), val = 20+1:7 )

...

merge( x, y, by="key" )

   key val.x val.y
1   2    12    22
2   3    13    23
3   3    13    26
4   3    14    23
5   3    14    26
6  NA    15    21
7  NA    15    27
8  NA    16    21
9  NA    16    27

As one should expect, there are now four lines with key value '3',
because the key '3' appears twice both in x and in y. According to the
logic of merge, a row should be produced in the output for each pairing
of a row from x and a row from y where the values of 'key' are equal.

However, the 'NA' values are treated exactly the same way. It seems that
'merge' considers the pairing of lines with 'NA' in both 'key' columns
an allowed match. IMHO, this runs against the convention that two NAs
are not considered equal. ('NA==NA' does not evaluate to 'TRUE'.)

Is might be more consistent if merge did not include any rows into the
output with an "NA" in the key column.

Maybe, one could add a flag argument to 'merge' to switch between this
behaviour and the current one? A note in the help page might be nice, too.

Splus (versions 8.0, 7.0, and 6.2) gives:

  > merge( x, y, by="key" )

    key val.x val.y
  1   2    12    22
  2   3    13    23
  3   3    14    23
  4   3    13    26
  5   3    14    26
Is that what you expect?  There is no argument
to Splus's merge to make it include the NA's
in the way R's merge does.  Should there be such
an argument?

----------------------------------------------------------------------------
Bill Dunlap
Insightful Corporation
bill at insightful dot com

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