moment.k<-function(x,
k = 2,
center = mean,
denominator = c("n", "n-1")
) {
n <- length(na.omit(x))
mp <- center(x)
sum( (x-mp)^k / (eval(parse(text = denominator[1]))) )
}
.moment.m2 <- function(x) {
moment.k(x,2)
}
.moment.m3 <- function(x) {
moment.k(x,3)
}
.moment.m4 <- function(x) {
moment.k(x,4)
}
#Test case
#moment.k(c(1,2,2,3,3,3),2)
#var(c(1,2,2,3,3,3))*5/6
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