Convert a Factor to a Numeric Variable
DESCRIPTION:
Converts a factor to a numeric variable.
USAGE: 
fac2num.factor(x, numeric.levels, coerce = TRUE)
REQUIRED ARGUMENTS:
x:
an object that inherits from class "factor".
OPTIONAL ARGUMENTS:
numeric.levels:
a vector giving the numeric values to be assigned to the levels of x. If numeric levels are not specified and levels(x) can be coerced to numeric values, these values are used levels. If numeric levels are not specified, levels(x) cannot be coerced to numeric and coerce is T the values 1:length(levels(x)) are used. Otherwise an error is generated.
coerce:
logical value for whether to coerce non-numeric levels to integers.
VALUE:
A numeric vector, with the factor levels replaced by numeric values.
WARNING:
The numeric levels are matched with the order of levels(x). The order of levels(x) determined internally by S-PLUS (typically as sort(unique(x)), and may not correspond to the order of appearance in the factor. For example, a factor with the levels "-" and "+", has levels(x) of c("+","-"). The numeric levels must be supplied in the corresponding order. However, if the factor is an ordered factor, with "-" < "+", this order is preserved by S-PLUS. See the example below.
EXAMPLES: 
f <- factor(rep(c("+","0","-"), 5))
ff <- ordered(rep(c("+","0","-"), 5), c("-","0","+"))
fac2num(f)
fac2num(ff)
fac2num(f, num = c( -5, 0, 5))
fac2num(ff, num = c( -5, 0, 5))