Mapping levels

mapLevels produces a map with information on levels and/or internal integer codes. As such can be conveniently used to store level mapping when one needs to work with internal codes of a factor and later transfrorm back to factor or when working with several factors that should have the same levels and therefore the same internal coding.

misc, manip
mapLevels(x, codes=TRUE, sort=TRUE, drop=FALSE, combine=FALSE, …)
mapLevels(x) <- value

object whose levels will be mapped, look into details


boolean, create integer levelsMap (with internal codes) or character levelsMap (with level names)


boolean, sort levels of character x, look into details


boolean, drop unused levels


boolean, combine levels, look into details

additional arguments for sort


levelsMap or listLevelsMap, output of mapLevels methods or constructed by user, look into details


mapLevels() returns “levelsMap” or “listLevelsMap” objects as described in levelsMap and listLevelsMap section.

Result of mapLevels<- is always a factor with remapped levels or a “list/data.frame” with remapped factors.


mapLevels function was written primarly for work with “factors”, but is generic and can also be used with “character”, “list” and “data.frame”, while “default” method produces error. Here the term levels is also used for unique character values.

When codes=TRUE integer “levelsMap” with information on mapping internal codes with levels is produced. Output can be used to transform integer to factor or remap factor levels as described bellow. With codes=FALSE character “levelsMap” is produced. The later is usefull, when one would like to remap factors or combine factors with some overlap in levels as described in mapLevels<- section and shown in examples.

sort argument provides possibility to sort levels of “character” x and has no effect when x is a “factor”.

Argument combine has effect only in “list” and “data.frame” methods and when codes=FALSE i.e. with character “levelsMaps”. The later condition is necesarry as it is not possible to combine maps with different mapping of level names and integer codes. It is assumed that passed “list” and “data.frame” have all components for which methods exist. Otherwise error is produced.

levelsMap and listLevelsMap

Function mapLevels returns a map of levels. This map is of class “levelsMap”, which is actually a list of length equal to number of levels and with each component of length 1. Components need not be of length 1. There can be either integer or character “levelsMap”. Integer “levelsMap” (when codes=TRUE) has names equal to levels and components equal to internal codes. Character “levelsMap” (when codes=FALSE) has names and components equal to levels. When mapLevels is applied to “list” or “data.frame”, result is of class “listLevelsMap”, which is a list of “levelsMap” components described previously. If combine=TRUE, result is a “levelsMap” with all levels in x components.

For ease of inspection, print methods unlists “levelsMap” with proper names. mapLevels<- methods are fairly general and therefore additional convenience methods are implemented to ease the work with maps: is.levelsMap and is.listLevelsMap; as.levelsMap and as.listLevelsMap for coercion of user defined maps; generic "[" and c for both classes (argument recursive can be used in c to coerce “listLevelsMap” to “levelsMap”) and generic unique and sort (generic from R 2.4) for “levelsMap”.


Workhorse under mapLevels<- methods is levels<-. mapLevels<- just control the assignment of “levelsMap” (integer or character) or “listLevelsMap” to x. The idea is that map values are changed to map names as indicated in levels examples. Integer “levelsMap” can be applied to “integer” or “factor”, while character “levelsMap” can be applied to “character” or “factor”. Methods for “list” and “data.frame” can work only on mentioned atomic components/columns and can accept either “levelsMap” or “listLevelsMap”. Recycling occurs, if length of value is not the same as number of components/columns of a “list/data.frame”.

See Also

factor, levels and unclass

  • mapLevels
  • mapLevels.default
  • mapLevels.factor
  • mapLevels.character
  • mapLevels.list
  • print.levelsMap
  • print.listLevelsMap
  • is.levelsMap
  • is.listLevelsMap
  • as.levelsMap
  • as.listLevelsMap
  • .checkLevelsMap
  • .checkListLevelsMap
  • "[.levelsMap"
  • "[.listLevelsMap"
  • c.levelsMap
  • c.listLevelsMap
  • unique.levelsMap
  • sort.levelsMap
  • mapLevels<-
  • mapLevels<-.default
  • mapLevels<-.factor
  • mapLevels<-.character
  • mapLevels<-.list
  • mapLevels<

## --- Integer levelsMap ---

(f <- factor(sample(letters, size=20, replace=TRUE)))
(mapInt <- mapLevels(f))

## Integer to factor
(int <- as.integer(f))
(mapLevels(int) <- mapInt)
all.equal(int, f)

## Remap levels of a factor
(fac <- factor(as.integer(f)))
(mapLevels(fac) <- mapInt) # the same as levels(fac) <- mapInt
all.equal(fac, f)

## --- Character levelesMap ---

f1 <- factor(letters[1:10])
f2 <- factor(letters[5:14])

## Internal codes are the same, but levels are not

## Get character levelsMaps and combine them
mapCha1 <- mapLevels(f1, codes=FALSE)
mapCha2 <- mapLevels(f2, codes=FALSE)
(mapCha <- c(mapCha1, mapCha2))

## Remap factors
mapLevels(f1) <- mapCha # the same as levels(f1) <- mapCha
mapLevels(f2) <- mapCha # the same as levels(f2) <- mapCha

## Internal codes are now "consistent" among factors

## Remap characters to get factors
f1 <- as.character(f1); f2 <- as.character(f2)
mapLevels(f1) <- mapCha
mapLevels(f2) <- mapCha

## Internal codes are now "consistent" among factors

Documentation reproduced from package gdata, version 2.18.0, License: GPL-2

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