numeric(n = 0) as.numeric(x) is.numeric(x)
numeric creates a real vector of length n. The vector
is filled with zeros.
as.numeric attempts to coerce its argument to numeric type
(either integer or real).
is.numeric returns TRUE if its argument is of type real
or type integer and FALSE otherwise.
as.numeric for factors yields the codes underlying the factor
levels.
as.numeric(c("2","B"))
as.numeric(factor(5:10))