Apply a function to each consecutive pairs in a vector.

diff.func <- function(x, func)
{ 
    ## Purpose: Apply a function to each consecutive pairs in a vector. 
    ## Arguments:
    ##   x: a vector
    ##   func: a function that operature on two elements (of "x")
    ## Return: a vector, whose length is length(x) - 1, that looks like (where N=length(x))
    ##    func(x[2], x[1]), func(x[3], x[2]), func(x[4], x[3]), ..., func(x[N], x[N-1]).
    ## Author: Feiming Chen, Date:  9 Feb 2017, 11:25
    ## ________________________________________________
    
    ## embed in 2-dim space to make consecutive pairs
    x2 <- embed(x, 2)                  # N - 1 pairs
    ## apply a function to each consecutive paris (reversely)
    x3 <- apply(x2, 1, func)
    x3
}
if (F) {                                # Unit Test
    diff.func(1:5, diff)                # -1 -1 -1 -1   
    diff.func(1:5, mean)                # 1.5 2.5 3.5 4.5
}