Calculating cumulative mean of recent observations - r

Question

My dataset has as features: players IDs, weeks and points.
I want to calculate the mean of points for previous weeks, but not all past weeks, just to the last 5 or less (if the current week is smaller than 5).
Example: For player_id = 5, week = 7, the result will be the average of POINTS for player_id = 5 and weeks 2, 3, 4, 5 and 6.
The following code already does the average for all previous week, so I need an adaptation to make it for just 5 previous week.
player_id<-c(rep(1,30),rep(2,30),rep(3,30),rep(4,30),rep(5,30))
week<-1:30
points<-round(runif(150,1,10),0)
mydata<- data.frame(player_id=player_id,week=rep(week,5),points)
mydata<-mydata %>%
group_by(player_id) %>% # the group to perform the stat on
arrange(week) %>% # order the weeks within each group
mutate(previous_mean = cummean(points) ) %>% # for each week get the
cumulative mean
mutate(previous_mean = lag(previous_mean) ) %>% # shift cumulative
mean back one week
arrange(player_id) # sort by player_id

Answer 1

HAVB's approach is great, but depending on what you want, here is another. This approach is adapted from this answer to a different question, but changed for your circumstances:
library(dplyr)
library(zoo)
# set the seed for reproducibility
set.seed(123)
player_id<-c(rep(1,30),rep(2,30),rep(3,30),rep(4,30),rep(5,30))
week<-1:30
points<-round(runif(150,1,10),0)
mydata<- data.frame(player_id=player_id,week=rep(week,5),points)
roll_mean <- function(x, k) {
result <- rollapplyr(x, k, mean, partial=TRUE, na.rm=TRUE)
result[is.nan(result)] <- NA
return( result )
}
mydata<- data.frame(player_id=player_id,week=rep(week,5),points)
mydata<-mydata %>%
group_by(player_id) %>%
arrange(week) %>%
mutate(rolling_mean = roll_mean(x=lag(points), k=5) ) %>%
arrange(player_id)
Then we can look at a subset to show it worked:
mydata[mydata$player_id %in% 1:2 & mydata$week %in% 1:6, ]
# A tibble: 12 x 4
# Groups: player_id [2]
player_id week points rolling_mean
<dbl> <int> <dbl> <dbl>
1 1 1 4 NA
2 1 2 8 4.000000
3 1 3 5 6.000000
4 1 4 9 5.666667
5 1 5 9 6.500000
6 1 6 1 7.000000
7 2 1 10 NA
8 2 2 9 10.000000
9 2 3 7 9.500000
10 2 4 8 8.666667
11 2 5 1 8.500000
12 2 6 5 7.000000
So we can see at each time t, rolling_mean for player i will be the mean of the points observations for player i at times {t - 1, ..., min(1, t - 5)}.

Answer 2

You can use slice to select just the last 5 weeks for each group. Try this:
player_id<-c(rep(1,30),rep(2,30),rep(3,30),rep(4,30),rep(5,30))
week<-1:30
points<-round(runif(150,1,10),0)
mydata<- data.frame(player_id=player_id,week=rep(week,5),points)
library(dplyr)
mydata <- mydata %>%
group_by(player_id) %>% # the group to perform the stat on
arrange(week) %>% # order the weeks within each group
slice( (n()-4):n() ) %>% # "slice" the last 5 rows (weeks) of every group
mutate(previous_mean = cummean(points) ) %>% # for each week get the cumulative mean
mutate(previous_mean = lag(previous_mean) ) %>% # shift cumulative mean back one week
arrange(player_id) # sort by player_id
The line
slice( (n()-4):n() )
selects rows within the range [(last row - 4) : last row], for each group
EDIT: To avoid trouble when the current week is less than 5, use an ifelse statement to validate:
mydata %>%
group_by(player_id) %>% # the group to perform the stat on
arrange(week) %>% # order the weeks within each group
slice(ifelse(n() < 5, 1:n(), n()-4):n()) %>% # "slice" the last 5 rows (weeks) of every group
mutate(previous_mean = cummean(points) ) %>% # for each week get the cumulative mean
mutate(previous_mean = lag(previous_mean) ) %>% # shift cumulative mean back one week
arrange(player_id) # sort by player_id