### Name: factor
### Title: Factors
### Aliases: factor ordered is.factor is.ordered as.factor as.ordered
###   is.na<-.factor Math.factor Ops.factor Summary.factor Ops.ordered
###   addNA
### Keywords: category NA

### ** Examples

(ff <- factor(substring("statistics", 1:10, 1:10), levels=letters))
as.integer(ff)  # the internal codes
factor(ff)      # drops the levels that do not occur
ff[, drop=TRUE] # the same, more transparently

factor(letters[1:20], labels="letter")

class(ordered(4:1)) # "ordered", inheriting from "factor"

## suppose you want "NA" as a level, and to allow missing values.
(x <- factor(c(1, 2, NA), exclude = NULL))
is.na(x)[2] <- TRUE
x  # [1] 1    <NA> <NA>
is.na(x)
# [1] FALSE  TRUE FALSE

## Using addNA()
Month <- airquality$Month
table(addNA(Month))
table(addNA(Month, ifany=TRUE))