Create Category from Discrete Data
DESCRIPTION:
Creates or tests for categorical objects (vectors with a "levels" attribute). This function is deprecated; factor or ordered are now preferred.
USAGE: 
category(x, levels=<<see below>>, labels=as.character(levels),
         ordered=F, exclude=NULL)
is.category(x)
as.category(x)
REQUIRED ARGUMENTS:
x:
data vector. Missing values (NAs) and Infs are allowed.
OPTIONAL ARGUMENTS:
levels:
vector of levels for the category; by default, the sorted unique values in x (that are not in exclude if exclude is not NULL). If levels is supplied an value in x but not in levels will be NA in the result. The argument exclude is ignored when levels is supplied. Missing values (NAs) and special values (NaNs and Infs) are allowed.
labels:
vector of character strings to use as labels for the levels of the category. The lengths of labels and levels must match.
ordered:
if TRUE, the attribute ordered is added to the return vector with value TRUE.
exclude:
a vector of values to be excluded from forming levels. Any value that appears in both x and exclude will be NA in the result and it will not appear in the default levels attribute.
VALUE:
vector of integers with levels attribute, indicating which level the category takes on for each data value. Any data value that does not match a value in levels is coded in the output vector as NA. Think of the return value as a subscript into the levels attribute.

is.category returns TRUE if x is a category object (has "levels"), and returns FALSE otherwise.

as.category returns x, if x is a category, category(x) otherwise.

SEE ALSO:
factor , ordered , levels , table , tapply , cut , tabulate .
EXAMPLES: 
category(occupation)  # "doctor", "lawyer", etc.


category(occupation, exclude=NA)
# NAs in the vector rather than a level for NAs


# make readable labels
occ <- category(occupation,level=c("d","l"),
    label=c("Doctor","Lawyer"))


# turn category into character vector
occ.char <- levels(occ)[occ]


colors <- category(color,c("red","green","blue"))
table(colors)  #table counting occurrences of colors