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
"All statements in this message represent the opinions of the author and do
not necessarily reflect Insightful Corporation policy or position."
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