nlevels

nlevels is a function in the R programming language that returns the number of levels in a factor or ordered factor. It provides a concise way to determine the size of the categorical domain of a variable. The argument x is typically a factor or an object that can be treated as one; nlevels returns an integer representing how many distinct levels exist in x. Conceptually, it is equivalent to length(levels(x)) since levels(x) yields the vector of all factor levels.

Usage and behavior: The function is part of base R and is primarily used when working with factor variables in data frames and statistical models. If x is a factor, nlevels(x) equals the number of elements in levels(x). If x is not a factor, the function is not intended for non-factor inputs and may return an error.

Examples: nlevels(factor(c("low","high","low"))) returns 2. nlevels(ordered(c("A","B","A"))) returns 2.

Related functions: levels(x) retrieves the vector of level labels; as.factor or factor can convert other data to factors; is.factor checks if an object is a factor. See also: unique and length as alternative ways to measure categories.

Notes: The function is part of the base stats package in R; for documentation, see the base R reference, where ?nlevels provides the official description and examples.