R help - May 2012 - function to calculate a SMAPE (Symmetric mean absolute percentage error) to avoid the possibility of an inflation caused by zero values in the series

Dear Reseacher,


i find this modified version of SMAPE at pag 13 formula "msSMAPE" to 
to
avoid the possibility of an inflation caused by zero values in the series
using a Si component in the denominator of the symmetric MAPE

http://www.stat.iastate.edu/preprint/articles/2004-10.pdf

I wrote a function but I wish eventually correct error, change or improve
with your suggestions. Please feel free to add comments or remarks because
I am not a mathematicians and probably
i wrong to write this function

Thanks in advance
Gianni



SMAPEper <- function(x,y){mean((200*(abs(x-y)))/(x+y))}
obs <- c(10,20,0,40)
pred <-c(5,6,0,8)


SMAPEper(obs,pred)



msSMAPE <- function(obs,pred){
    M <- numeric(length(obs))

    for (i in 2:length(obs)) {
         n <- i-1
         M[i] <- mean(obs[1:n])
    }
    sum1 <- (abs(obs[1]-pred[1]))/(0.5*(abs(pred[1])+abs(obs[1])))

    for (i in 2:length(obs)) {
        n <- i-1
         sum2 <- 0
         for (k in 1:n) {sum2 <- sum2+abs(obs[k]-M[k])}
         S <- sum2/n
         sum1 <-
sum1+(abs(obs[i]-pred[i]))/(0.5*(abs(pred[i])+abs(obs[i]))+S)
    }

    my.SMAPE <- sum1/length(obs)
    return(my.SMAPE)
}

msSMAPE(obs,pred)

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