R devel - Aug 2010 - No [[<-.factor()

Should there be a [[<-.factor() that either throws
an error or acts like [<-.factor() to avoid making
an illegal object of class factor?

  > z <- factor(c("Two","Two","Three"),
levels=c("One","Two","Three"))
  > z
  [1] Two   Two   Three
  Levels: One Two Three
  > str(z)
   Factor w/ 3 levels "One","Two","Three": 2 2 3
  > z[[2]] <- "One"
  > str(z) # the .Data part is now character
   Factor w/ 3 levels "One","Two","Three": 2 One 3
  > z
  [1] <NA> <NA> <NA>
  Levels: One Two Three
  > z[2] <- "One"
  Error in class(x) <- cx : adding class "factor" to an invalid
object

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

>>>>> "WD" == William Dunlap <wdunlap at
tibco.com>
>>>>>     on Wed, 25 Aug 2010 17:31:27 -0700 writes:
    WD> Should there be a [[<-.factor() that either throws
    WD> an error or acts like [<-.factor() to avoid making
    WD> an illegal object of class factor?

Yes, one or the other.
Note that both `[<-` and `[[<-` are "Primitive" and do some
"informal dispatch" in addition to the formal method dispatch,
so there's no need for an explicit '[[<-.factor' method
definition.
For factors, it seems rational and "expectable" that 
double bracket behaves as single bracket when *one* element is
assigned.  
   z[[2:3]] <- c("Two, Three")
however  should continue to give an error.

Martin Maechler, ETH Zurich

    >> z <-
factor(c("Two","Two","Three"),
levels=c("One","Two","Three"))
    >> z
    WD> [1] Two   Two   Three
    WD> Levels: One Two Three
    >> str(z)
    WD> Factor w/ 3 levels "One","Two","Three":
2 2 3
    >> z[[2]] <- "One"
    >> str(z) # the .Data part is now character
    WD> Factor w/ 3 levels "One","Two","Three":
2 One 3
    >> z
    WD> [1] <NA> <NA> <NA>
    WD> Levels: One Two Three
    >> z[2] <- "One"
    WD> Error in class(x) <- cx : adding class "factor" to an
invalid object

    WD> Bill Dunlap
    WD> Spotfire, TIBCO Software
    WD> wdunlap tibco.com 

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