Create a new column based multiple rows of another column

Question

I have a dataframe as this

> df<-data.frame(index=c(1,2,3,4,5,6),value=c(2,3,5,8,11,12))
> df
    index value
1     1     2
2     2     3
3     3     5
4     4     8
5     5    11
6     6    12

I want to create a new column which equals to the the sum of three adjacent values of column value indexed by column index, that is

> df_res
  index value res
1     1     2 NA
2     2     3 10
3     3     5 16
4     4     8 24
5     5    11 31
6     6    12 NA

The second row of res is the sum of (2,3,5), third sum(3,5,8) etc. (the first and last row of res do not matter, and I temporarily set it as NA)

How can I get it done in R?

Answer 1

If you use data.table:

library(data.table)
setDT(df)
df[,res:=value+shift(value,1)+shift(value,1,type="lead")]

Answer 2

you can use dplyr and roll_sum to do:

df %>% 
  mutate(v2 = roll_sum(value, 3,fill = NA))

which gives:

  index value v2
1     1     2 NA
2     2     3 10
3     3     5 16
4     4     8 24
5     5    11 31
6     6    12 NA

Answer 3

df$res <- sapply(df$index, function(index) 
    ifelse(index > 1 & index < nrow(df),sum(df$value[(index - 1):(index + 1)]), NA))

   index value res
1     1     2  NA
2     2     3  10
3     3     5  16
4     4     8  24
5     5    11  31
6     6    12  NA

Answer 4

Using head and tail:

df$res <- df$value + c(tail(df$value, -1), NA) + c(NA, head(df$value, -1))

df
#   index value res
# 1     1     2  NA
# 2     2     3  10
# 3     3     5  16
# 4     4     8  24
# 5     5    11  31
# 6     6    12  NA

Or using zoo:

df$res <- zoo::rollsum(df$value, 3, na.pad = TRUE)