The function factor
is used to encode a vector as a factor (the
terms ‘category’ and ‘enumerated type’ are also used for
factors). If argument 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.
1 2 3 4 5 6 7 8 9 10 11 12 
x 
a vector of data, usually taking a small number of distinct values. 
levels 
an optional vector of the values (as character strings)
that 
labels 
either an optional character vector of labels for the
levels (in the same order as 
exclude 
a vector of values to be excluded when forming the
set of levels. This should be of the same type as 
ordered 
logical flag to determine if the levels should be regarded as ordered (in the order given). 
nmax 
an upper bound on the number of levels; see ‘Details’. 
... 
(in 
ifany 
(only add an 
The type of the vector x
is not restricted; it only must have
an as.character
method and be sortable (by
sort.list
).
Ordered factors differ from factors only in their class, but methods and the modelfitting 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
(which will happen for excluded values) 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 nooperation
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 (prints as <NA>
); by default, this is the last level.
If NA
is a level, the way to set a code to be missing (as
opposed to the code of the missing level) is to
use is.na
on the lefthandside of an assignment (as in
is.na(f)[i] < TRUE
; indexing inside is.na
does not work).
Under those circumstances missing values are currently printed as
<NA>
, i.e., identical to entries of level NA
.
is.factor
is generic: you can write methods to handle
specific classes of objects, see InternalMethods.
Where levels
is not supplied, unique
is called.
Since factors typically have quite a small number of levels, for large
vectors x
it is helpful to supply nmax
as an upper bound
on the number of unique values.
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
and unique
(!anyDuplicated(.)
) entries. If argument 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 an ordered
factor 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.
addNA
modifies a factor by turning NA
into an extra
level (so that NA
values are counted in tables, for instance).
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 transform a factor f
to
approximately 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 some anomalies associated with factors that have
NA
as a level. It is suggested to use them sparingly, e.g.,
only for tabulation purposes.
There are "factor"
and "ordered"
methods for the
group generic Ops
which
provide methods for the Comparison operators,
and for the min
, max
, and
range
generics in Summary
of "ordered"
. (The rest of the groups and the
Math
group 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. Collation is done by the levels of the operands: if both operands are ordered factors they must have the same level set.
In earlier versions of R, storing character data as a factor was more space efficient if there is even a small proportion of repeats. However, identical character strings now share storage, so the difference is small in most cases. (Integer values are stored in 4 bytes whereas each reference to a character string needs a pointer of 4 or 8 bytes.)
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24  (ff < factor(substring("statistics", 1:10, 1:10), levels = letters))
as.integer(ff) # the internal codes
(f. < 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"
z < factor(LETTERS[3:1], ordered = TRUE)
## and "relational" methods work:
stopifnot(sort(z)[c(1,3)] == range(z), min(z) < max(z))
## 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))

Questions? Problems? Suggestions? Tweet to @rdrrHQ or email at ian@mutexlabs.com.
All documentation is copyright its authors; we didn't write any of that.