Applying function to consecutive subvectors of equal size

Question

I am looking for a nice and fast way of applying some arbitrary function which operates on vectors, such as sum, consecutively to a subvector of consecutive K e

Answer 1

Try this:

library(zoo)
rollapply(v, 3, by = 3, sum, partial = TRUE, align = "left")
## [1]  6 15 15

or

apply(matrix(c(v, rep(NA, 3 - length(v) %% 3)), 3), 2, sum, na.rm = TRUE)
## [1]  6 15 15

Also, in the case of sum the last one could be shortened to

colSums(matrix(c(v, rep(0, 3 - length(v) %% 3)), 3))

Answer 2

As @Chase said in a comment, you can create your own grouping variable and then use that. Wrapping that process into a function would look like

myapply <- function(v, fun, group_size=1) {
    unname(tapply(v, (seq_along(v)-1) %/% group_size, fun))
}

which gives your results

> myapply(v, sum, group_size=3)
[1]  6 15 15

Note this does not require the length of v to be a multiple of the group_size.

Answer 3

You could try this as well. This works nicely even if you want to include overlapping intervals, as controlled by by, and as a bonus, returns the intervals over which each value is derived:

library (gtools)
v2 <- running(v, fun=sum, width=3, align="left", allow.fewer=TRUE, by=3)

v2
1:3 4:6 7:8 
  6  15  15