Check if a vector's values are all the same

is.constant  <- function(x)
{ 
    ## Purpose: Check if a vector's values are all the same. 
    ## Arguments:
    ##   x: a vector (numeric, character, logical, etc.)
    ## Return: a logical value tha is TRUE if and only if all the values are the same (constant). 
    ## Author: Feiming Chen, Date: 13 Apr 2017, 14:59
    ## ________________________________________________
    y <- rep.int(x[1], length(x))
    isTRUE(all.equal(x, y))
}
if (F) {                                # Unit Test
    is.constant(c(T, T, F))             # F
    is.constant(c(F, F, F))             # T
    is.constant(c(1, 1, 2))             # F
    is.constant(c(2, 2, 2))             # T
    is.constant(c("a", "a", "b"))       # F
    is.constant(c("b", "b", "b"))       # T
}