Cumulative average by group

Question

I want to compute trailing mean GPA by Name from the table below at each index. For example at index 9, I want to have the mean GPA for C from indices 3, 6, 9 and likewise for

Answer 1

Using dplyr:

g <- data.frame(Name=rep(c("A","B","C"),3),
            GPA=c(5,6,7,5,6,6,7,6,3))

g %>%
group_by(Name) %>%
mutate(cumu = lag(cummean(GPA), n = 0))

Output:

Source: local data frame [9 x 3]
Groups: Name [3]

Name   GPA     cumu
     
1      A     5 5.000000
2      B     6 6.000000
3      C     7 7.000000
4      A     5 5.000000 
5      B     6 6.000000
6      C     6 6.500000
7      A     7 5.666667
8      B     6 6.000000
9      C     3 5.333333