| factor {base} | R Documentation |
The function factor is used to encode a vector as a factor (the
terms ‘category’ and ‘enumerated type’ are also used for
factors). If ordered is TRUE, the factor levels are
assumed to be ordered.
For compatibility with S there is also a function ordered.
is.factor, is.ordered, as.factor and as.ordered
are the membership and coercion functions for these classes.
factor(x = character(), levels = sort(unique.default(x), na.last = TRUE),
labels = levels, exclude = NA, ordered = is.ordered(x))
ordered(x, ...)
is.factor(x)
is.ordered(x)
as.factor(x)
as.ordered(x)
x |
a vector of data, usually taking a small number of distinct values. |
levels |
an optional vector of the values that x might
have taken. The default is the set of values taken by x,
sorted into increasing order. |
labels |
either an optional vector of labels for the
levels (in the same order as levels after removing those in
exclude), or a character string of length 1. |
exclude |
a vector of values to be excluded when forming the
set of levels. This should be of the same type as x, and
will be coerced if necessary. |
ordered |
logical flag to determine if the levels should be regarded as ordered (in the order given). |
... |
(in ordered(.)): any of the above, apart from
ordered itself. |
The type of the vector x is not restricted.
Ordered factors differ from factors only in their class, but methods and the model-fitting functions treat the two classes quite differently.
The encoding of the vector happens as follows. First all the values
in exclude are removed from levels. If x[i] equals
levels[j], then the i-th element of the result is
j. If no match is found for x[i] in levels,
then the i-th element of the result is set to NA.
Normally the ‘levels’ used as an attribute of the result are
the reduced set of levels after removing those in exclude, but
this can be altered by supplying labels. This should either
be a set of new labels for the levels, or a character string, in
which case the levels are that character string with a sequence
number appended.
factor(x, exclude=NULL) applied to a factor is a no-operation
unless there are unused levels: in that case, a factor with the
reduced level set is returned. If exclude is used it should
also be a factor with the same level set as x or a set of codes
for the levels to be excluded.
The codes of a factor may contain NA. For a numeric
x, set exclude=NULL to make NA an extra
level ("NA"), by default the last level.
If "NA" is a level, the way to set a code to be missing is to
use is.na on the left-hand-side of an assignment.
Under those circumstances missing values are printed as <NA>.
is.factor is generic: you can write methods to handle
specific classes of objects, see InternalMethods.
factor returns an object of class "factor" which has a
set of integer codes the length of x with a "levels"
attribute of mode character. If ordered is true
(or ordered is used) the result has class
c("ordered", "factor").
Applying factor to an ordered or unordered factor returns a
factor (of the same type) with just the levels which occur: see also
[.factor for a more transparent way to achieve this.
is.factor returns TRUE or FALSE depending on
whether its argument is of type factor or not. Correspondingly,
is.ordered returns TRUE when its
argument is ordered and FALSE otherwise.
as.factor coerces its argument to a factor.
It is an abbreviated form of factor.
as.ordered(x) returns x if this is ordered, and
ordered(x) otherwise.
The interpretation of a factor depends on both the codes and the
"levels" attribute. Be careful only to compare factors with
the same set of levels (in the same order). In particular,
as.numeric applied to a factor is meaningless, and may
happen by implicit coercion. To “revert” a factor f to
its original numeric values, as.numeric(levels(f))[f] is
recommended and slightly more efficient than
as.numeric(as.character(f)).
The levels of a factor are by default sorted, but the sort order may well depend on the locale at the time of creation, and should not be assumed to be ASCII.
There are "factor" and "ordered" methods for the
group generic Ops, which
provide methods for the Comparison operators. (The rest of the
group and the Math and
Summary groups generate an error as
they are not meaningful for factors.)
Only == and != can be used for factors: a factor can
only be compared to another factor with an identical set of levels
(not necessarily in the same ordering) or to a character vector.
Ordered factors are compared in the same way, but the general dispatch
mechanism precludes comparing ordered and unordered factors.
All the comparison operators are available for ordered factors. Sorting is done by the levels of the operands: if both operands are ordered factors they must have the same level set.
Storing character data as a factor is more efficient storage if
there is even a small proportion of repeats. On a 32-bit machine
storing a string of n bytes takes
28 + 8*ceiling((n+1)/8)
bytes whereas storing a factor code takes 4 bytes. (On a 64-bit
machine 28 is replaced by 56 or more.) Only if they were computed
from the same values (or in some cases read from a file: see
scan) will identical strings share storage.
Chambers, J. M. and Hastie, T. J. (1992) Statistical Models in S. Wadsworth & Brooks/Cole.
[.factor for subsetting of factors.
gl for construction of “balanced” factors and
C for factors with specified contrasts.
levels and nlevels for accessing the
levels, and unclass to get integer codes.
(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], label="letter")
class(ordered(4:1)) # "ordered", inheriting from "factor"
## suppose you want "NA" as a level, and to allowing missing values.
(x <- factor(c(1, 2, "NA"), exclude = ""))
is.na(x)[2] <- TRUE
x # [1] 1 <NA> NA, <NA> used because NA is a level.
is.na(x)
# [1] FALSE TRUE FALSE
factor()