[R] NA and logical indexes

[Berwin A Turlach]

G'day Sebastian,

On Fri, 28 Nov 2008 16:35:13 -0600
"Sebastian P. Luque" <spluque at gmail.com> wrote:

> On Fri, 28 Nov 2008 22:01:15 -0000 (GMT),
> (Ted Harding) <Ted.Harding at manchester.ac.uk> wrote:
>
> > NA can seem to have a bewildering logic, but it all becomes clear if
> > you interpret NA as "value unkown".
> > [...] 
> > Clear? ( :)) Hoping this helps, Ted.
> 
> Crystal!  It's something that one needs to be extra careful with when
> playing with logicals.

Or, rather, if you expect that your code one day has to handle data in
which values from which you construct logicals might be missing (NA),
then it might be worthwhile to think carefully about this case and how
you want to handle missing values.  In your case, do you want to
return the rows where foo$A is missing or not?  Both is possible:

R> foo <- data.frame(A=gl(2, 5, labels=letters[1:2]), X=runif(10))
R> foo$A[1] <- NA
R> foo[!is.na(foo$A) & foo$A == "b", ]  
   A         X
6  b 0.8418619
7  b 0.6833322
8  b 0.1582648
9  b 0.4431542
10 b 0.7603365
R> foo[is.na(foo$A) | foo$A == "b", ]  
      A         X
1  <NA> 0.8922147
6     b 0.8418619
7     b 0.6833322
8     b 0.1582648
9     b 0.4431542
10    b 0.7603365

Cheers,

	Berwin

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