如何计算每行最后N行的滚动平均值

Question

Hi I have a df and How to calculate rolling mean for previous N rows ,In my case it is 3

df <- data.frame(X=c(24,NA,NA,45,NA,20,24,10,40,20,20), Y=c(10,NA,14,14,14,10,NA,10,14,14,14) )

I tried to get o/p as below

library(data.table)
library(zoo)
df[, Rolling.Average := rollmeanr(X, 3, fill = NA), by = Y]

o/p:

X   Y  Rolling.Average
24  10  NA
NA  NA  NA
NA  14  NA
45  14  NA
NA  14  45
20  10  NA
24  NA  NA
10  10  20
40  14  NA
20  14  40
20  14  30

Error: k <= n is not TRUE Any modifications in above code,please suggest Thanks

Answer 1

The error is thrown as you have less that 3 rows for the "NA" group -- which data.table seems to interpret as a group.

So, your data is expected to have at least three entries for each by-group. Then the code works (I have added a row below to your example below):

df <- data.frame(X=c(24,NA,NA,45,NA,20,24,10,40,20,20,20),
                 Y=c(10,NA,14,14,14,10,NA,10,14,14,14,NA))

library(data.table)
library(zoo)
df <- as.data.table(df)
df[, Rolling.Average := rollmeanr(X, k = 3, fill = NA), by = Y]
print(df)
#     X  Y Rolling.Average
# 1: 24 10              NA
# 2: NA NA              NA
# 3: NA 14              NA
# 4: 45 14              NA
# 5: NA 14              NA
# 6: 20 10              NA
# 7: 24 NA              NA
# 8: 10 10              18
# 9: 40 14              NA
#10: 20 14              NA
#11: 20 14              NA
#12: 20 NA              NA

Or, you can exclude the NA s of the by-clause by something like:

df[!is.na(Y), Rolling.Average := rollmeanr(X, k = 3, fill = NA), by = Y]

Which yields the same output. It does not match your expected output, but I do not really understand how one should arrive at that.