[R] Adding logical vectors

[David Huffer]

When adding several logical vectors I expect each vector will be
coerced to integers and these vectors will then be added.

That doesn't always seem to be the case.

For example:

  > ( f1 <- as.factor ( sample ( "x" , 25 , rep = T ) ) )
   [1] x x x x x x x x x x x x x x x x x x x x x x x x x
  Levels: x
  > ( f2 <- as.factor ( sample ( "y" , 25 , rep = T ) ) )
   [1] y y y y y y y y y y y y y y y y y y y y y y y y y
  Levels: y
  > ( f3 <- as.factor ( sample ( "z" , 25 , rep = T ) ) )
   [1] z z z z z z z z z z z z z z z z z z z z z z z z z
  Levels: z
  >
  > is.na ( f1 [ sample ( 1:25 , 4 ) ] ) <-
  + is.na ( f2 [ sample ( 1:25 , 4 ) ] ) <-
  + is.na ( f3 [ sample ( 1:25 , 4 ) ] ) <- TRUE
  >
  > ##  this returns a numeric vector:
  >
  > is.na ( f1 ) + is.na ( f2 ) + is.na ( f3 )
   [1] 0 0 0 0 0 1 1 0 0 0 1 0 0 1 0 0 1 0 1 2 2 0 1 0 1
  >
  > ##  but this returns a logical vector
  >
  > !is.na ( f1 ) + !is.na ( f2 ) + !is.na ( f3 )
   [1]  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE
   [9]  TRUE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE  TRUE
  [17] FALSE  TRUE FALSE  TRUE FALSE  TRUE FALSE  TRUE
  [25] FALSE
  >

Can someone please explain why the returned value is a logical
vector when I use the not operator but a numeric vector when I
don't.

What is special about the !is.na? it returns an object of class
logical just like the is.na function:

  > all.equal ( class ( !is.na ( f1 ) ) , class ( is.na ( f1 ) ) )
  [1] TRUE
  >

Thanks!