Generating a moving sum variable in R

Question

I suspect this is a somewhat simple question with multiple solutions, but I\'m still a bit of a novice in R and an exhaustive search didn\'t yield answers that spoke well to wha

Answer 1

If DF is the input three-column data frame then use ave with rollapplyr from zoo. Note that we use a width of k+1 and then drop the k+1st element from the sum so that the current value of x is excluded and only the remaining k values are summed:

library(zoo)

k <- 5
roll <- function(x) rollapplyr(x, k+1, function(x) sum(x[-k-1]), fill = NA)
transform(DF, xSyrsum = ave(x, country, FUN = roll))

which gives:

   country year x xSyrsum
1        A 1980 9      NA
2        A 1981 3      NA
3        A 1982 5      NA
4        A 1983 6      NA
5        A 1984 9      NA
6        A 1985 7      32
7        A 1986 9      30
8        A 1987 4      36
9        B 1990 0      NA
10       B 1991 4      NA
11       B 1992 2      NA
12       B 1993 6      NA
13       B 1994 3      NA
14       B 1995 7      15
15       B 1996 0      22