levels {base} | R Documentation

## Levels Attributes

### Description

`levels` provides access to the levels attribute of a variable. The first form returns the value of the levels of its argument and the second sets the attribute.

### Usage

    levels(x)
    levels(x) <- value

### Arguments

`x` | an object, for example a factor.
`value` | A valid value for `levels(x)`. For the default method, `NULL` or a character vector. For the `factor` method, a vector of character strings with length at least the number of levels of `x`, or a named list specifying how to rename the levels.

### Details

Both the extractor and replacement forms are generic and new methods can be written for them. The most important method for the replacement function is that for `factor`s.

For the factor replacement method, a `NA` in `value` causes that level to be removed from the levels and the elements formerly with that level to be replaced by `NA`.

Note that for a factor, replacing the levels via `levels(x) <- value` is not the same as (and is preferred to) `attr(x, "levels") <- value`.

The replacement function is primitive.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

### See Also

`nlevels`, `relevel`, `reorder`.

### Examples

    ## assign individual levels
    x <- gl(2, 4, 8)
    levels(x)[1] <- "low"
    levels(x)[2] <- "high"
    x

    ## or as a group
    y <- gl(2, 4, 8)
    levels(y) <- c("low", "high")
    y

    ## combine some levels
    z <- gl(3, 2, 12, labels = c("apple", "salad", "orange"))
    z
    levels(z) <- c("fruit", "veg", "fruit")
    z

    ## same, using a named list
    z <- gl(3, 2, 12, labels = c("apple", "salad", "orange"))
    z
    levels(z) <- list("fruit" = c("apple","orange"),
                      "veg"   = "salad")
    z

    ## we can add levels this way:
    f <- factor(c("a","b"))
    levels(f) <- c("c", "a", "b")
    f

    f <- factor(c("a","b"))
    levels(f) <- list(C = "C", A = "a", B = "b")
    f

[Package base version 3.3.0 Index]