| 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 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.
factor(x = character(), levels, labels = levels,
exclude = NA, ordered = is.ordered(x), nmax = NA)
ordered(x, ...)
is.factor(x)
is.ordered(x)
as.factor(x)
as.ordered(x)
addNA(x, ifany = FALSE)
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 (unique)
labels for the levels (in the same order as |
exclude |
a vector of values to be excluded when forming the
set of levels. This may be factor with the same level set 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 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
(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 without
NAs 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, since R version 3.4.0, excluding non-existing
character levels is equivalent to excluding nothing, and when
exclude is a character vector, that is
applied to the levels of x.
Alternatively, exclude can be factor with the same level set as
x and will exclude the levels present in exclude.
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 left-hand-side 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").
Undocumentedly for a long time, factor(x) loses all
attributes(x) but "names", and resets
"levels" and "class".
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 (sometimes faster) 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).
.valid.factor(object) checks the validity of a factor,
currently only levels(object), and returns TRUE if it is
valid, otherwise a string describing the validity problem. This
function is used for validObject(<factor>).
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.
(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
## More rational, since R 3.4.0 :
factor(c(1:2, NA), exclude = "" ) # keeps <NA> , as
factor(c(1:2, NA), exclude = NULL) # always did
## exclude = <character>
z # ordered levels 'A < B < C'
factor(z, exclude = "C") # does exclude
factor(z, exclude = "B") # ditto
## Using addNA()
Month <- airquality$Month
table(addNA(Month))
table(addNA(Month, ifany = TRUE))