r - How can I create a sum of every n numbers? -


this question has answer here:

i have dataframe column: 

1 1 1 1 2 2 2 2 3 3 3 3

and want sum every n numbers (say 4). intended output:

4 8 12

how can this? know how rollsum, wanted know if there function this?

x <- data.frame(value = c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3)) x$group = rep(1:(nrow(x)/4), each=4)  tapply(x$value, x$group, fun = sum)

the hardest part here assigning groups. see ?rep details there.


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