[Rd] subset and missing value indexing
Adaikalavan Ramasamy
ramasamy at cancer.org.uk
Tue Aug 17 19:55:58 CEST 2004
Out of curiosity, is this a bug or a feature or "==" ?
m <- matrix( 1:12, nc=4 )
f <- c("A", NA, "B", "A")
f == "A"
[1] TRUE NA FALSE TRUE
m[ , f == "A"] # equivalent to m[ , c(1, NA, 4) ]
[,1] [,2] [,3]
[1,] 1 NA 10
[2,] 2 NA 11
[3,] 3 NA 12
m[ , which(f == "A")]
[,1] [,2]
[1,] 1 10
[2,] 2 11
[3,] 3 12
In arguments section of help("which") it says that
'NA's are allowed and omitted (treated as if 'FALSE').
After some thinking, I think this might be due to subsetting using index
that includes missing value. help("[") appears not to say what happens
when one of the indexing element is a missing value, only that the index
can be logical ( and NA is logical ).
Is there any reason for allowing NA when subsetting ?
Regards, Adai
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