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factor

For representing categorical data with a finite number of options (levels)

Create

Action	Code	Details
From vector	factor(x)
From vector, with renamed levels	factor(x, levels=1:3, labels=LETTERS[1:3]
Generate by pattern	gl(5, 2, labels=LETTERS[1:5])
Generate factor of interactions with another	interaction(f1, f2)
Combine factors	factor(c(as.character(x), as.character(y)), union(levels(x), levels(y)))

Test

Action	Code	Details
Is factor	is.factor(x)
Contains value v	v %in% f
Contains NA value (not as level)	anyNA(f)
Contains NA (as <NA> level)	NA %in% f	anyNA(f) does not work!
Contains level named v	v %in% levels(f)
Contains <NA> level	anyNA(levels(f))

Extract

Action	Code	Details
Levels	levels(f)
Number of levels	nlevels(f)
Levels, as factor, and in order	factor(levels(f), levels(f))
Used levels, as factor, and in order	sort(unique(f))	Useful for generating reference categorical newdata for predict()

Derive

Action	Code	Details
Remap categories / values	x = factor(c('a1', 'b1', 'a2', 'c1')) factor(x, levels=levels(x), labels=c('a', 'a', 'b', 'c'))

Alter levels

Action	Code	Details
Add NA as a factor level	f = addNA(f)
Add NA as a factor level if needed (NAs present)	f = addNA(f, ifany = TRUE)
Drop specific factor levels	droplevels(f, exclude=X)
Drop factor levels except specific	droplevels(f, except=X)
Drop unused factors levels	factor(f)

Shrink

Action	Code	Details
Subset, drop unused levels	f[1:2, drop=TRUE]

Grow

Join

Action	Code	Details
Combine factors	c(x, y)	Resulting factor has levels that are a union of the original levels

Convert

Action	Code	Details
To vector of strings	as.character(x)
To vector of level indices	as.numeric(x)
To vector of level indices	as.integer(x)
To ordered factor	as.ordered(x)