Count the number of distinct values in a vector or distinct rows in a data frame

N.levels <- function(x)
{ 
    ## Purpose: Count the number of distinct values in a vector or distinct rows in a data frame
    ## Arguments:
    ##    x: a vector (numeric or string), or a data frame. 
    ## Return: a count for the distinct values/rows in the vector or data frame. 
    ## Author: Feiming Chen, Date: 20 Mar 2017, 10:58
    ## ________________________________________________

    sum(!duplicated(x))
}
if (F) {                                # Unit Test
    N.levels(c(1,2,1,3))                # 3
    N.levels(c("a", "b", "a"))          # 2
    x <- data.frame(a=c(1,2,1), b=c(1, 2, 1))
    N.levels(x) # 2
}