R help - Sep 2001 - Characters vectors, NA's and "" in merges

I often use merge with dataframes that contain character vectors which have
elements that are sometimes "NA" (meaning the string NA, not the same
thing,
obviously, as NA in a numeric or factor vector). For example, the stock ticker
for Nabisco was "NA". Unfortunately (for me), it seems like merge
insists on
inserting "NA" for missing values. My question: Is there some way
around this?
Here is a simple example:
> version
         _                   
platform sparc-sun-solaris2.6
arch     sparc               
os       solaris2.6          
system   sparc, solaris2.6   
status                       
major    1                   
minor    3.0                 
year     2001                
month    06                  
day      22                  
language R                   
> a <- data.frame(x = 1:4)
> b <- data.frame(x = 1:3, y = c("NA", "a",
"b"))
> merge(a, b, all.x = TRUE)
  x  y
1 1 NA
2 2  a
3 3  b
4 4 NA

Rows 1:3 are what I expect them to be. Row 4 is "wrong" in the sense
that
dataframe b did not contain a row for x = 4. Of course, there is a sense that
*any* value, including "", that is placed in row 4 is potentially
misleading. Perhaps I am misunderstanding the meaning of "NA" in a
character
vector (i.e., I am not allowed to have "real" values that are that
string).

If there were some way (an "nomatch" argument?) that the user could
specify
what missing values are used for character strings, then I would be
fine. Again, I suspect that my real problem is not understanding how to specify
"NA" -- meaning Nabisco's ticker symbol -- in a character vector.

Any suggestions would be much appreciated.

Dave Kane
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On Wed, 26 Sep 2001, David Kane  <David Kane wrote:
> I often use merge with dataframes that contain character vectors which have
> elements that are sometimes "NA" (meaning the string NA, not the
same thing,
> obviously, as NA in a numeric or factor vector). For example, the stock
ticker
> for Nabisco was "NA". Unfortunately (for me), it seems like merge
insists on
> inserting "NA" for missing values. My question: Is there some way
around this?
> Here is a simple example:
>
> > version
>          _
> platform sparc-sun-solaris2.6
> arch     sparc
> os       solaris2.6
> system   sparc, solaris2.6
> status
> major    1
> minor    3.0
> year     2001
> month    06
> day      22
> language R
>
> > a <- data.frame(x = 1:4)
> > b <- data.frame(x = 1:3, y = c("NA", "a",
"b"))
Take a look.  b$y is a factor with levels "a" and "b", and a
missing first
value.
> > merge(a, b, all.x = TRUE)
>   x  y
> 1 1 NA
> 2 2  a
> 3 3  b
> 4 4 NA
>
> Rows 1:3 are what I expect them to be. Row 4 is "wrong" in the
sense that
> dataframe b did not contain a row for x = 4. Of course, there is a sense
that
> *any* value, including "", that is placed in row 4 is potentially
> misleading. Perhaps I am misunderstanding the meaning of "NA" in
a character
> vector (i.e., I am not allowed to have "real" values that are
that string).
That is the correct answer. Because you asked for all.x=TRUE, you
got a missing value there in row 4 col 2.
> If there were some way (an "nomatch" argument?) that the user
could specify
> what missing values are used for character strings, then I would be
> fine. Again, I suspect that my real problem is not understanding how to
specify
> "NA" -- meaning Nabisco's ticker symbol -- in a character
vector.
You cannot avoid it being taken as the missing value, AFAIK.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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