Function

absolute.sum<-function(n)  ## a function that adds from -n to n
{
     a<-0
     for(i in -n:n)
{
             if(i>=0) a<-a+i  ## if i is positive, add i to a
                else a<-a+(-i)  ## if i negative, add the absolute value -i to a
}
return(a)
}

> absolute.sum(100)

Tip: looping is very expensive in R. Always avoid looping if possible

absolute.sum1<-function(n) ## this is another version of faster implementation
{ return(sum(abs(-n:n))) }

absolute.sum(1000000)  ## this will take a while
absolute.sum1(1000000) ## this will take less than a second